Dual redundant active/active brake-by-wire architecture

ABSTRACT

The braking control system provides dual redundant control of hydraulically operated wheel braking for an aircraft. A primary hydraulic system provides hydraulic power for normal operation of the plurality of wheel brakes, and a secondary hydraulic system provides hydraulic power for alternate operation of the plurality of wheel brakes. A control unit is provided for controlling brake pressure communicated to the wheel brakes through the primary and secondary hydraulic systems, and a monitor channel is operatively connected to the primary hydraulic system for detecting faults in the primary and secondary hydraulic systems and for selecting between the primary and secondary hydraulic systems for providing braking pressure. The monitor channel detects occurrence of loss of pressure in the primary hydraulic system, if any brake has unwanted pressure applied, and if a fault is detected on the primary or secondary channels that affects more than one wheel brake on each landing gear. The primary hydraulic system comprises at least one primary hydraulic fluid control channel and at least one secondary hydraulic fluid control channel, the primary and secondary fluid channels being redundant and partitioned among the plurality of wheel brakes so that even if both the primary and secondary channels fail to apply pressure, braking will be lost to only a portion of the wheel brakes and the loss will be in a symmetrical pattern, and the secondary hydraulic system comprises at least one primary hydraulic fluid control channel and at least one secondary hydraulic fluid control channel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to aircraft landing gear brakingsystems, and more particularly concerns an improved system forprotection against inadvertent braking, and multiply redundant separatedbrake control channels.

2. Description of Related Art

Automatic braking systems have been commonly provided on commercialaircraft to aid the deceleration of the aircraft upon landing. As thesize and complexity of aircraft have increased, the automatic brakingsystems have also become more complex and computerized. Modem anti-skidsystems incorporated into aircraft braking systems commonly optimizebraking efficiency by adapting to runaway conditions and other factorswhich affect braking in order to optimize deceleration typicallycorresponding to the level of brake pressure selected by the pilot.

A catastrophic failure mode can occur in a conventional brake-by-wirecontrol system that results in uncommanded brake application on one ormore wheels during takeoff of the aircraft. Since uncommanded brakingduring takeoff can have serious consequences, and at the very least canresult in unnecessary and accelerated wear to the braking system, it isdesirable to configure the braking system to reduce the possibility ofthese undesirable results. The overriding primary consideration is, ofcourse, safety, although considerations of reliability are alsosignificant.

High performance digital brake-by-wire control systems have beendeveloped and installed on several aircraft including light commercialjet transports and modern business jets that use brake pressure feedbackand enhanced built-in-test capability. Brake torque control is also usedto further enhance brake control. Such digital brake control systemshave achieved excellent braking performance over all runway conditions,and in RTO (Refused Take Off) and landing configurations. Withsophisticated brake control algorithms, optimum braking performance isassured regardless of conditions, and the same software configurationcan be used with assured range of brake and hydraulic configurations.

Two specific catastrophic failure modes that need to be addresses by anaircraft braking control system architecture are: a) the inadvertentapplication of any brake during the takeoff roll, and b) the completeloss of braking. The problem of inadvertent application of any brakeduring the takeoff roll sets the following design requirements: 1) nosingle failure shall result in the application of any brake during takeoff; and 2) the probability of any combination of failures leading toany brake being applied during take off shall be extremely improbable(less than 1×10⁻⁹). The second catastrophic hazard, the loss of allbraking, sets the following design requirements: 1) no single failureshall lead to loss of all braking; and 2) the probability of anycombination of failures leading to loss of all braking shall beextremely improbable (less than 1×10⁻⁹). These high performancerequirements preclude the exclusive use of software. In addition,another commonly known braking control architecture has the disadvantagethat the active brake control hydraulic fluid channels are connected toa single coil within the brake control valve, which provides a singlepoint of failure that can result in catastrophic failure in the event offailure at that point.

Redundancy is typically achieved by use of a master or monitor channelthat is used to monitor the operational status of hydraulic fluidbraking control channels, and the monitor channel can command a firstcontrol channel to turn off and a second control channel to commencecontrol, for example. Another method of redundancy management uses twocontrol channels with one active control channel, and a second, inactivecontrol channel in standby mode. When the active channel shuts down, thestandby channel takes over control. However, both the master-slave andthe active-standby systems can permit a single failure within the masteror the active channel to cause a major breakdown in the redundancymanagement system.

In addition, loss of braking can also occur as a result of the antiskidfunction, requiring accounting for the probability that normal,alternate, emergency, and ultimate brake systems will be depressurisedby a single failure of the anti-skid system. Loss of braking can occurowing to incorrect antiskid activity as a result of control systemfailure or loss of aircraft power. Another significant failure is theloss of gear retraction braking, which could allow a wheel with a loosetire tread to enter the wheel well while spinning. The hardwiredinterlock used to prevent application of brakes during take-off,typically conflicts with the requirement to stop the wheels during climbwhen the thrust levers are advanced.

Furthermore, the need to preclude asymmetric braking as a result of theloss of braking, or extra braking on one main landing gear set thefollowing design requirements: 1) combinations of failures leading tothe loss of all braking on either main landing gear shall be improbable(1×10⁶); and 2) combinations of failures leading to extra braking oneither main landing gear shall be improbable (1×10⁻⁶). Touchdown andaquaplaning protection is provided by comparing wheel speeds with thegroundspeed signal from the Air Data Inertial Reference Units (ADIRU).Typically any main gear aft wheel that is at a velocity 50 knots or morebelow the ADIRU groundspeed value is given a brake release signal.Undesired asymmetrical release of brakes can result from a false ADIRUsignal, or from unwanted pressure being applied to any brake.

A need therefore continues to exist for an improved aircraft landinggear braking control system. The present invention addresses these andother needs.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides for abraking control system for dual redundant control of hydraulicallyoperated wheel brakes of aircraft landing gear providing protectionagainst inadvertent braking, and separation of braking control throughprimary and secondary braking control channels using an interface withdual coil brake control valves. The braking control system is safe,reliable, maintainable, lightweight, and affordable, and provides for aredundant brake-by-wire control architecture using a primary dualredundant brake-by-wire braking system, and a secondary dual redundantanalog brake-by-wire system. Positive hydraulic system selection betweenthe normal primary and alternate secondary hydraulic braking systems isperformed using solenoid operated shutoff valves (SOSV). The primarybraking system control of a center landing gear, if one is present, issplit between right and left pedals, with the front axle of center gearlanding controlled by left pedals, and the aft axle of center landinggear controlled by right pedals, and locked wheel protection isperformed on a tandem basis rather than on an axle basis to preventfault propagation. Alternate braking is performed on a paired wheelbasis through the alternate hydraulic system. The primary braking systemincludes pressure and antiskid control performed using dual coil servovalves with pressure feedback, autobrake control employing primary brakesystem servo valves, and an equal load distribution provided by pressurefeedback control. Emergency braking is also provided, allowing brakingwhen all electrical power generation and all hydraulic power generationis lost. Parking brake and ultimate braking modes are also provided,using hydraulic power stored in accumulators.

The present invention accordingly provides for a braking control systemfor dual redundant control of hydraulically operated wheel braking foran aircraft having landing gear that can move between a retractedposition and an actuated position, the landing gear having a pluralityof wheels and a corresponding plurality of wheel brakes for saidplurality of wheels, and a plurality of brake pedals for controllingoperation of braking of said plurality of said wheels. In a presentlypreferred embodiment, a primary hydraulic system is connected in fluidcommunication with the plurality of wheel brakes for providing hydraulicpower for normal operation of the plurality of wheel brakes in a normalbraking mode, and a secondary hydraulic system is connected in fluidcommunication with the plurality of wheel brakes for providing hydraulicpower for alternate operation of the plurality of wheel brakes in analternate braking mode. A control unit is provided for controlling brakepressure communicated to the wheel brakes through the primary andsecondary hydraulic systems, and a monitor channel is operativelyconnected to the primary hydraulic system for detecting faults in theprimary and secondary hydraulic systems and for selecting between theprimary and secondary hydraulic systems for providing braking pressure.In a presently preferred aspect, the monitor channel detects occurrenceof loss of pressure in the primary hydraulic system, if any brake hasunwanted pressure applied, and if a fault is detected on the primary orsecondary channels that affects more than one wheel brake on eachlanding gear.

In a presently preferred aspect of the invention, the primary hydraulicsystem comprises at least one primary hydraulic fluid control channeland at least one secondary hydraulic fluid control channel, the primaryand secondary fluid channels being redundant and partitioned among theplurality of wheel brakes so that even if both the primary and secondarychannels fail to apply pressure, braking will be lost to only a portionof the wheel brakes and the loss will be in a symmetrical pattern, andthe secondary hydraulic system comprises at least one primary hydraulicfluid control channel and at least one secondary hydraulic fluid controlchannel. In one currently preferred embodiment, wheel braking power isprovided by common fluid channels to adjacent ones of the right and leftmain landing gear front and aft wheel brakes to provide protectionagainst asymmetrical wheel braking, and primary and secondary fluidchannels control all four wheels of the center landing gear. In apresently preferred aspect, the wheel braking power is provided bycommon fluid channels to the wheel brakes of the center landing gear onan axle pair basis. Typically, the primary hydraulic system comprisesthree primary hydraulic fluid control channels and three secondaryhydraulic fluid control channels that operate simultaneously andindependently, and are arranged in redundant pairs of primary andsecondary fluid channels, and each pair primary and secondary fluidchannels controls the same four wheels.

In a presently preferred embodiment, the secondary hydraulic systemprovides pressure for alternate braking using dual, independent, closedloop analog control circuits, and the secondary hydraulic systemprovides dual redundant analog brake-by-wire control in the alternatebraking mode for the main and center landing gears of the aircraft. Thesecondary hydraulic system preferably comprises a plurality ofaccumulators for providing an alternate supply of hydraulic power, andthis alternate supply of hydraulic power is provided for an emergencybraking mode in the event that both the primary hydraulic system and thesecondary hydraulic system are depressurized. This alternate supply ofhydraulic power is similarly provided for an ultimate braking modeproviding braking pressure to a plurality of the wheel brakes, and thealternate supply of hydraulic power is also provided for a parking brakemode providing braking pressure to a plurality of the wheel brakes.

The present invention preferably provides for first and second solenoidoperated shut-off valves operatively connected to the primary andsecondary hydraulic systems, respectively, and to the control unit forselecting operation of one of the primary and secondary hydraulicsystems, the first and second solenoid operated shut-off valves beingconfigured to operate in a mutually exclusive manner to position selectbetween operation of the primary and secondary hydraulic systems withoutthe possibility of having both systems pressurized at the same time.

In another presently preferred aspect, the control unit comprises thrustlever switches, and the solenoid operated shut-off valves areimplemented through the thrust lever switches to positively preventpressure from the primary hydraulic system and secondary hydraulicsystem being applied to the normal or alternate brake metering systemsduring take off. Typically a landing gear lever is provided controllingretraction of the landing gear, and dual redundant switches on thelanding gear lever to bypass the thrust lever switches to stop thewheels during climb when the thrust levers are advanced to enable wheelbraking upon retraction of the landing gear. In another preferredaspect, each of the primary brake hydraulic fluid control channelsreceives a software independent signal that initiates retraction brakingfor three seconds or until the nose landing gear is up and locked,whichever happens sooner, and an anti-skid function of the normalbraking mode is inhibited during retraction braking.

The control unit in a presently preferred embodiment comprises aplurality of servo control valves controlled by corresponding dualsolenoid coils for controlling the operation of the wheel brakes,respectively, and the thrust levers comprise dual thrust lever switchesthat break both power and ground to the first and second solenoidoperated shut-off valves for the primary hydraulic system and secondaryhydraulic systems when a thrust lever is advanced. Further, the controlunit can additionally comprise a plurality of sensors for sensing theposition of each brake pedal, such as dual redundant switches that breakboth power and ground to the first and second solenoid operated shut-offvalves, such that depression of either brake pedal opens the first andsecond solenoid operated shut-off valve for the active hydraulic system.In a currently preferred aspect, the first and second solenoid operatedshut-off valves also turn off hydraulic power to the servo controlvalves during flight. In another presently preferred aspect, the controlunit further comprises sensor means for determining brake pedalapplication and for generating a pedal application signal indicatingactuation of the wheel braking system when the brake pedal has beenapplied, and can also include means for sensing weight on the wheel forgenerating a brake inhibit signal when weight is not applied on thewheel.

The present invention provides, in a currently preferred embodiment thatthe servo control valves for controlling the operation of the wheelbrakes controlled by dual solenoid coils, and in a further preferredaspect, are also controlled with pressure feedback. Antiskid control isthus preferably provided on each wheel brake utilizing the dual coilservo control valves with pressure feedback. The control unit preferablycomprises a pressure sensor mounted downstream of each servo controlvalve for detecting asymmetrical braking due to unwanted pressureapplied to any wheel brake, whereby the monitor channel can select thealternate hydraulic fluid system.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description and the accompanyingdrawings, which illustrate by way of example the features of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the braking control system of thepresent invention;

FIG. 2 is a schematic diagram of the hydraulic system brake sourceselection logic of the braking control system of FIG. 1;

FIG. 3 is a functional block diagram of the brake-by-wire system of thebraking control system of FIG. 1;

FIG. 4 is a diagram illustrating the control partitioning for the normalprimary hydraulic braking system of the braking control system of FIG.1;

FIG. 5 is a functional block diagram for the normal, primary brakecontrol and the alternate, secondary brake control of the brakingcontrol system of FIG. 1;

FIG. 6 is a diagram illustrating the control partitioning for thealternate, secondary hydraulic braking system of the braking controlsystem of FIG. 1;

FIG. 7 is a chart of the commanded brake pressure vs. the correspondingbrake pedal displacement of the braking control system of FIG. 1;

FIG. 8 is a schematic diagram of the parking brake subsystem of thebraking control system of FIG. 1;

FIG. 9 is a schematic diagram of the gear retraction braking subsystemof the braking control system of FIG. 1;

FIG. 10 is a schematic diagram of the brake signal partitioning of thebraking control system of FIG. 1;

FIG. 11 is a schematic diagram of the emergency braking hydraulic fluiddistribution scheme of the braking control system of FIG. 1;

FIG. 12A is a schematic diagram of the park braking hydraulic fluiddistribution scheme of the braking control system of FIG. 1;

FIG. 12B is a schematic diagram of the park/ultimate braking valve ofFIG. 12A;

FIG. 13A is a top plan view of a solenoid operated shutoff valveaccording to the present invention;

FIG. 13B is a side view of the solenoid operated shutoff valve of FIG.13A; and

FIG. 14 is a sectional view of the solenoid operated shutoff valve ofFIG. 13A and 13B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Catastrophic failure of aircraft landing gear wheel brakes can resultfrom uncommanded brake application on one or more wheels during takeoff,and from the complete loss of braking. Design requirements forsubstanatially eliminating the probability of such catastrophic brakingfailure involve eliminating single point of hardware or softwarefailures that can result in such catastrophic failures. The presentinvention provides an architecture that uses two completely independentmeans of applying brakes. The normal means of stopping the aircraft usesthe primary hydraulic system and a dual redundant, closed loop, digitalpressure control system to meter pressure to the normal brake system.The alternate means uses a software independent, closed loop, analogcontrol system to meter secondary hydraulic system pressure to thealternate brake system. Both normal and alternate means of applyingbrakes operate continuously and autonomously without dependence oninformation from one to the other.

As is illustrated in the drawings, the invention is accordinglyembodiment in a braking control system for dual redundant control ofhydraulically operated wheel brakes of aircraft landing gear providingprotection against inadvertent braking, and separation of brakingcontrol through primary and secondary braking control channels using aninterface with dual coil brake control valves, such as for a brake andsteering control system for an aircraft such as the Airbus A340-500/600.

The present invention provides for a braking control system with abraking and steering control unit (BSCU), which is typically anintegrated, digital control unit. The BSCU provides dual redundantdigital brake-by-wire, autobrake control, and Nose Wheel Steeringcapability in the normal mode. The brake control unit also provides dualredundant, analog brake-by-wire for the alternate braking mode. Brakecontrol includes independent antiskid protection, locked wheelprotection, touchdown/hydroplane protection, gear retract braking, andcomprehensive built-in-test (BIT). In addition to these controlfunctions, the control unit provides ARINC 429 communication withinterfacing aircraft systems. The braking and steering control systemcomplies with all of the braking and steering JAR and FAR requirements.

Referring to FIG. 1, the braking control system 30 provides dualredundant, digital brake-by-wire control in a normal or primary brakingmode, and dual redundant analog brake-by-wire control in an alternate orsecondary braking mode for the left main landing gear 32, the right mainlanding gear 34 and the center landing gear 36 of the aircraft. Nosewheel steering control can also be provided as an integrated part of thebraking and steering control unit. The system provides braking for theeight main gear wheels and the four center gear wheels in the normal,alternate, and emergency braking modes. Ultimate braking and a parkbrake are provided to the eight-wheels of the main landing gears.

The braking system operates on the primary (normal) 38 and secondary(alternate) 40 hydraulic systems. The primary hydraulic system providespower for normal braking, and the secondary hydraulic system providespower for alternate braking. Two six-liter accumulators 42 foremergency, ultimate, and park braking augment the secondary hydraulicsystem. A hydraulic check valve 44 provides isolation of theaccumulators.

Referring to FIG. 2, hydraulic system selection is performed using twosolenoid operated shut-off valves (SSOV's or SOV's) 46. These valves arearranged to operate in a mutually exclusive manner to provide positivesystem selection without the possibility of having both systemspressurized at the same time, which could damage the brakes. The valvesincorporate a soft turn-on feature to avoid undesired hydraulicimpulses.

Two 3-way solenoid operated shutoff valves are installed one in theprimary hydraulic system and one in the secondary hydraulic system forhydraulic system selection, fault isolation, and to minimize hydraulicsystems leakage in flight. The envelope dimensions of a system shutoffvalve and a cross-section are depicted in FIGS. 13A, 13B, and 14,respectively. The-solenoid valve assembly consists of a solenoid valvecontrolling a spool-sleeve valve 48. The solenoid valve portion is a twoball normally closed type. One ball 50 connects or isolates supplypressure to the spool position control port 52 while the other ball 53performs the same type of function for return pressure. With thesolenoid de-energized, pressure holds the supply pressure ball 50against the seat sealing off the control port 52. A small pintle 54pushes the return pressure ball 53 off its seat and connects the controlport to return pressure. When the solenoid is energized, the plungerpushes the pin 56 which seats the return pressure ball 53 and throughthe pintle pushes the supply pressure ball off its seat. In thisposition the control port is connected to supply pressure and isolatedfrom return pressure.

The second stage is a spool-sleeve valve 58 pressure driven in bothdirections. The spring 60 holds the spool to a position, opening thecylinder port 62 to return and blocking the pressure port (closedposition) when the solenoid valve is de-energized or/and system pressureis not present. The spring also provides a fail energized function. Whenpressure is present it acts on the area of the bullet piston 66 andexerts a 200 pound closing force holding the spool to in the closedposition. In this de-energized condition, the control pressure chamber68 is vented to return through the solenoid valve. The spring endchamber 70 (opposite the control pressure chamber) is always vented toreturn. When the solenoid valve is energized, pressure is ported to thecontrol pressure chamber on the spool and acts on the main spool area.This generates an opening force of 400 pounds. This force overcomes theconstant 200 pound force and strokes the spool to the open position witha net force of 200 pounds. With the spool in this position, the pressureport is open to cylinder and the return port is blocked. Internalorificing within the spool can be sized to provide damping and spoolstroke rates that avoid undesirable fluid momentum pressure spikes(water hammer).

With reference to FIG. 3, normal braking is performed through a dualredundant, digital brake-by-wire architecture. Pressure and antiskidcontrol is performed on each wheel using dual coil servo valves 80 withpressure feedback. This allows each channel to independently control thepressure of each brake commensurate with the level commanded by thebrake pedal position sensors. The dual coil servo valves also provideadequate channel separation for redundancy management. The brake-by-wirealgorithm provides smooth brake application without torque spiking orbrake grab. As installed for an aircraft preferably includes a wheelspeed transducer 82 for each wheel brake 84 of a wheel 85 of theaircraft, for measuring wheel speed and generating wheel speed signalsthat are a function of the rotational speed of the brake wheel. Thewheel speed signal is typically converted to a signal representing thevelocity of the aircraft by a velocity converter 86, and compared with adesired reference velocity in velocity comparator 88, to generate wheelvelocity error signals indicative of the difference between the wheelvelocity signals from each braked wheel and the reference velocitysignal. The output of the velocity comparator is referred to as slipvelocity (Vs) or velocity error. The velocity error signals are adjustedby a pressure bias modulator control means (PBM) integrator 90, thetransient control means 92, and compensation network 94, the outputs ofwhich are summed at summing junction 96 to provide an antiskid controlsignal 98 received by the command processor 100, typically amicroprocessor. The PBM integrator in the antiskid loop dictates themaximum allowable control pressure level during braking. The PBMintegrator is typically slower in response than other control parametersneeded to detect and control initial skid. When no skid is detected,this integrator allows full system pressure to the brakes.

The position of the aircraft brake pedal 102 operated by the pilot istypically read by a microcontroller 103 that generates a brake pedalcommand signal 104, from which a pressure application profile isdetermined. The command processor 100 receives the brake pedal commandsignal, the antiskid control signal 98 via feedback line 106, andpreferably also receives a locked wheel protection signal 108 indicatingwhether a wheel is locked, and a touchdown/hydroplaning protectionsignal 110, to guard against hydroplaning of a wheel on touchdown athigh speeds or on slippery runway surfaces at speeds above 50 knots. Ina currently preferred embodiment, the command processor operates on thelowest input of the locked wheel protection signal, the touchdownprotection signal, the pedal signal, and the antiskid signal. Thecommanded brake pressure signal output 112 of the command processor iscompared with the brake pressure feedback signal 114 from brake pressuresensor 116 by comparator 118, which generates an output pressure errorsignal 120.

In a currently preferred embodiment, the brake pressure error signalsare also adjusted by a proportional gain by proportional gain circuitry122, an intergral gain by integral gain circuitry 124, and adifferential gain by differential gain circuitry 125 that together forma PID control loop, and the outputs of which are summed at summingjunction 126 to provide an adjusted brake pressure signal 127. Theadjusted brake pressure signal is also typically amplified by valveamplifier 128 to provided an amplified brake control signal applied tothe brake control valve 80 that controls the application of pressurizedbrake fluid from system pressure 132 to the wheel brake. In a presentlypreferred embodiment, these functions can be performed by one or moremicroprocessors under appropriate software control, althoughalternatively these or analogous functions may be performed by suitablehardware components.

The system will provide differential braking capability for steering theaircraft with the nose gear free castoring from either the Captain's orFirst Officer's brake pedals. Four dual redundant brake pedalpotentiometers provide the Captain/First Officer pedal positioninformation to the braking computer. The pedal potentiometers provideCaptain/First Officer pedal position information to the brakingcomputer. Wheel speed information is derived from twelve, axle mounted,wheel speed tachometers. The wheel speed tachometers are variablereluctance devices. The outputs of the wheel speed tachometers areproportional to the rotational speed of the wheels.

Brake pressure information is derived from eighteen brake pressuretransducers. The pressure transducers are 4-20 ma current outputdevices. The current transducers were chosen because they significantlyreduce aircraft wiring, are highly reliable, and are relatively immuneto electrical interference.

System pressure availability is determined through two upstream pressureswitches. One pressure switch is connected to the primary hydraulicsystem the second is connected to the secondary hydraulic system. Apressure transducer in the secondary system downstream of the checkvalve provides accumulator pressure indication to the brake control andcockpit.

As is illustrated in FIG. 4, the braking control system preferablyconsists of six channels of brake control: three primaries 140 a, 140 b,140 c, and three secondaries 142 a, 142 b, 142 c. The six channels arearranged in redundant pairs of a primary and secondary, each paircontrolling the same four wheels. The primary and secondary systemsoperate simultaneously and independently. The scheme ensures that theloss of a redundant pair of control channels will not lead to asymmetricbraking. In FIG. 4, locked wheel pairs are shown by the double arrows.For the main gears, locked wheel protection is done on a tandem basisrather than on an axle basis. This maintains full separation betweenchannels to prevent the possibility of fault propagation.

The center gear is arranged so that the left brake pedals 144 controlthe front axle pair 146 and the right brake pedals 148 control the aftaxle pair 150. This is done to avoid cyclic torsional fatigue loading ofthe structure. Two redundant channels control all four wheels of thecenter gear. For the center gear, locked wheel protection is done on anaxle pair basis. The pairing arrangement for center gear can readily bechanged.

Referring to FIG. 5, autobrake control with five landing modedeceleration settings and an RTO (Refused Take Off) mode is provided forthe normal brake system. In FIG. 5, only a primary channel is shown,however the secondary channel is identical to the primary channel,except that the other coil of the servo valve is used. The normal systemservo valves 80 are used for autobrake pressure application. Pressurefeedback control is used in the autobrake control mode the same as inpedal braking mode to ensure equal load distribution amongst all thebrakes.

Alternate braking is performed on a paired wheel basis on the secondaryhydraulic system using a dual redundant, analog brake-by-wire pressurecontrol system. The proposed wheel pairing is the same as the lockedwheel pairing on the normal system. This allows a simple approach whereboth normal and alternate antiskid control can be provided by the samecontrol channel. A further advantage of this pairing is that duringoperation on slushy or icy runways, where a pair of tandem wheels tendsto encounter a rut or strip of ice, only the tandem pair is releasedinstead of the entire gear as would happen using axle pairing. FIG. 5shows how the antiskid commands from the primary channel and thepressure command from the alternate analog are combined by hardware inthe alternate control channel to produce the alternate servo valvecontrol command.

FIG. 6 shows the partitioning for the alternate brake system. The flightcrew will be unaware of any difference in system operation when usingalternate brakes or during switch over to the alternate from normal,except for the appropriate cockpit indication. Pedal application on thealternate system follows a pressure gain profile identical to that onthe normal brake control.

Emergency braking mode uses the accumulators on the secondary system andthe essential electrical bus. The emergency braking system allowsbraking when all electrical power generation is lost or both primary andsecondary hydraulic power generation is lost. In the emergency mode, thealternate braking system is used with antiskid protection turned off.Operation of the antiskid switch places all digital processors in thereset mode positively inhibiting the antiskid function to all brakes(shutting down brake control power supplies may be used if deemednecessary).

The emergency braking system provides braking to center landing gear andmain landing gear wheels. Seven pedal applications from brake contactpressure to 100 bars minimum can be made on the emergency brakingsystem. This performance is achieved by using the same accurate,brake-by-wire, pressure control algorithm as used in the alternatebraking mode, as illustrated in FIG. 7.

Referring to FIG. 8, the parking brake provides 175 bars to the eightmain landing gear brakes. A latching solenoid operated valve is used toprovide accumulator pressure from the secondary system to the eight mainlanding gear brakes. Pressure application is through the return path ofthe alternate braking servo valves. This method reduces component countby eliminating extra shuttle valves and provides reduced leakage andlonger parking time. Lap leakage of the alternate servo valves on themain gear is not a factor, since the 3 way Solenoid Operated Shut-OffValve (SOSV) will be in the pressure to brake mode, therefore reducingleakage dramatically. An isolation valve 152 in the pressure supply lineof the servo valves for the center gear blocks this leak path when thereturn line is pressurized. The park brake valve 154 limits brakepressure to 175 bars even when the accumulator is charged to 206 bars.

The secondary hydraulic system also provides pressure for the park brakevalve. Two six-liter accumulators back up the secondary hydraulicsystem. These accumulators will provide extended park pressure. Alatching solenoid park brake valve provides park pressure through thereturn port of the alternate servo valves. The position of the parkbrake valve is fed back to the control unit through a position switchthat is incorporated in the valve.

Return side check valves are provided to avoid any back flow due tovarious hydraulic failure modes. The braking and steering control unitactively monitors accumulator pressure. The brake control unit suppliespower and return signals to the two 3 way SOSV's (Solenoid OperatedShutoff Valves) that are connected to hydraulic system primary andhydraulic system secondary. The shutoff valves provide means ofcontrolling inlet pressure to the brake control valves. The approachprovides exclusive system selection along with isolation of faultyhydraulic components in case of a failure. The shutoff valves are alsoused to turn off hydraulic power to the servo valves during flight.Pressure switches located on the outlet of the primary and secondarysystem SOSV facilitate fault detection and isolation by the controlunit. The control unit commands and controls brake pressure throughtwelve dual coil brake control valves in the normal channel and eightdual coil brake control valves in the alternate control channel.

Each brake control servo valve is protected against hydraulic leaks witha volumetric fuse. A fuse is used between the brake control valve outletand the brake inlet. This prevents reservoir depletion in case of abroken line anywhere downstream of the brake control valve.

If both the normal and alternate braking systems are lost, the ultimatebraking feature provides 175 bars pressure to the eight main landinggear brakes to stop the aircraft. Ultimate braking is engaged byoperating the electrical park brake switch. The safety considerationsthat were used in establishing the architecture described above arediscussed below.

Referring to FIG. 12, the hydraulic accumulators for the parking brakewill be capable of maintaining parking brake pressure of 175 bar at alleight main landing gear wheels for a period of 12 hours. After initialbrake application, from the secondary hydraulic system, the parkingbrake pressure is maintained at 179 bar for a 12-hour period. The two 6liter accumulators are thus sufficient to comply with the requirementsfor parking/ultimate braking and for Emergency braking.

The park/ultimate brake valve 154 includes the following components: a)a latching three-way solenoid valve 164; b) a spring loaded piston 166;c) a three-way, metering spool valve 168; and d) a bias spring 170. Themagnetic circuit of the latching solenoid is designed to hold thesolenoid plunger in the energized de-energized position after removal ofelectrical power to the coil. The solenoid must be driven from one stateto the other. The valving element of the latching solenoid consists of aspring-loaded poppet metering on a hard steel seat. A three-wayproximity switch provides solenoid plunger position status. The pressuremetering spool is designed so that with no pressure applied, a biasspring holds the spool in a position that allows porting of thealternate brake valve module return line to the secondary system returnline port. When pressure is applied, the metering spool will move themain spool to provide brake pressure regulated at 175 bar. High overlapensures extremely low pressure to return leakage. Based on the parkingbrake valve leakage of 6 cm3/hr, the brake pressure will be at 178 barafter a 12-hour parking period of the aircraft on the two six literaccumulators initially pressurized to 206 bar.

Since a low pressure can be used to actuate the valve (<40 bar), thedesign ensures that the full accumulator pressure stays applied to theparking brake if accumulator pressure should drop below the 175 barlevel. Additionally the valve acts as a thermal relief valve in caseparking brake pressure exceeds a preset limit due to fluid expansion.

The strict performance requirements for the braking control systempreclude the exclusive use of software and hence the approach taken isto use hardware interlocks through the thrust levers 172 to positivelyprevent pressure from the primary or secondary hydraulic systems beingapplied to the normal or alternate brake metering systems during takeoff. FIG. 2 shows the right brake pedal and No. 1 thrust lever. The leftbrake pedal and No. 3 thrust lever are not shown but are similar to theright brake pedal and No. 1 thrust lever of FIG. 2. FIG. 2 also showsthe solenoid operated shutoff valves (SOSVs) used to block the pressurefrom the primary and secondary hydraulic systems if one or more thrustlevers is at or above the minimum take off thrust setting. The valvesare forced closed by supply pressure to minimize the probability ofsticking owing to contamination. Both primary and secondary hydraulicsystems are vented to low pressure when the solenoid valves are in theclosed position.

Dual switches 174 break both power and ground to the solenoid coil forthe primary and secondary systems when either the No. 1 or No. 3 thrustlever is advanced. The monitor channel detects disagreement between twoswitches mounted on the same lever. The switches are spring loaded tothe advanced position so that if a switch falls off its mounting, itwill fail safe. The pressure downstream of each solenoid valve ismonitored to detect mechanical failures. It is assumed that the case ofan engine ferry using the No. 1 or No. 3 engine position will be handledby procedure. This scheme positively prevents the application of brakeswith take-off thrust applied but it also introduces a performancedeficit during a brake pedal initiated Refused Take Off (RTO) stop.Flight crew procedures require the pedal brakes to be applied before thethrust levers are retarded and, without the addition of pedal logic, RTObrake application would be delayed. This shortcoming is avoided bysensing the position of each brake pedal using software independent,hard-wired switches. Depression of either brake pedal opens the valvefor the active hydraulic system as shown in FIG. 2. The switches aredual redundant, break both power and ground to both valves, and aremonitored for passive failures. In the event of the loss of allengine-driven generators, both valves move to the closed position.

As discussed above, the normal brake system consists of six channels ofbrake application: three primaries and three secondaries. Referring toFIG. 5, each of the four brake pedals, Captain's Right, Captain's Left,First Officer's Right, and First Officer's Left is equipped with aposition sensor 176. The monitor channel digital processor 178 comparesthe positions measured by each sensor to detect passive failures. Thepressure sensors measure the pressure at each brake to minimize theerror between brake pressure and pedal position so that equal pressuresare applied to all brakes. If one of the channels fails to commandpressure, the dual coil servo valve allows the other channel to controlthe brake pressure independently. The channels are partitioned so that,even if both the primary and its secondary channel fail to applypressure, braking will be lost to four brakes only and the loss will bein a symmetrical pattern. In the event that the primary hydraulic systemloses pressure, or any brake has unwanted pressure applied, or a faultis detected on the primary or secondary channels that affects more thanone brake on each landing gear, the monitor channel will switch to thesecondary hydraulic system as the source of braking pressure.

The braking control architecture described was evaluated against thefailure condition requirement for “loss of normal braking availability”.The failure probability requirement for this condition is 1×10⁻⁶ (perflight hour). The current architecture is based on an Active/Activecontrol where primary and secondary channels operate simultaneously andautonomously. This architecture has numerous benefits in redundancymanagement and system operation. The proposed architecture is flexibleenough to provide an Active/Standby control with more complex redundancymanagement in the BITE cards 180. Using this approach, BITE shuts down anormal channel that is determined to be faulty and operation continuesusing the second channel of the normal system without switching to thealternate brakes. This approach can further be augmented byincorporating dual element pressure transducers into the Active/Standbycontrol.

The secondary hydraulic system provides pressure to the alternate brakesystem using dual, independent, closed loop analog control circuits.Three primary and three secondary channels are also provided for dualredundant, independent means of applying brakes. As in the normalsystem, dual coil servo control valves are used to ensure that if onechannel fails to apply pressure, the remaining channel can applypressure to the commanded value.

In the event that both the primary and secondary hydraulic system aredepressurised, emergency braking can be accomplished using the brakeaccumulators installed in the alternate brake system. As a last resort,the ultimate braking system can be used to stop the aircraft byoperating the Park Brake switch. On the normal system, combining thepressure application and pressure reduction commands in softwareproduces the servo valve signals. On the alternate system, the anti-skidpressure reduction command for each brake pair is combined in hardwarewith the independent pressure command from the analog circuit. Thepartitioning of the channels ensures that a failure causing loss ofbrakes will be confined to four wheels only and will be distributedsymmetrically. This approach assumes that all digital control systemswill not fail simultaneously because of a specific set of systemconditions. It is believed that this theoretical event is not applicablefor small, relatively simple state machines such as brake controlsystems. The proposed approach has been successfully certified on manyaircraft models over the last twenty years. The provision of an AntiskidOff switch and an ultimate braking system diminishes the probability ofloss of all braking owing to an antiskid malfunction to below 1×10⁻⁹.

Another significant failure is the loss of gear retraction braking. Thiswould allow a wheel with a loose tire tread to enter the wheel wellwhile spinning The hardwired interlock used to prevent application ofbrakes during take-off, conflicts with the requirement to stop thewheels during climb when the thrust levers are advanced. Retractionbraking is enabled by using dual redundant switches on the landing gearlever 182 to bypass the thrust lever switches as shown in FIG. 2. Thelanding switches are monitored for passive failures. Also referring toFIG. 9, gear retract braking is applied only from the primary system andso the landing gear lever switches do not inhibit the secondary systemSOSV. This reduces the probability of the loss of alternate braking. Therequired probability for this event prohibits the use of a singleprocessor for applying brakes. Each of the normal brake control channelsreceives a software independent signal that initiates retractionbraking. A brake pressure profile is applied to each brake through thenormal servo valves for three seconds or until the nose landing gear isup and locked, whichever happens sooner. The anti-skid function of allnormal processors is inhibited during retraction braking.

The need to preclude asymmetric braking as a result of the loss of orextra braking on one main landing gear is another significantconsideration in the control of aircraft braking. The partitioningscheme used for normal and alternate anti-skid functions shown in FIG. 5prevents the loss of braking owing to incorrect antiskid activity asresult of control system failure or loss of aircraft power. The pedalswitches and sensors are dual redundant and are monitored. If one of thepedal sensors fails to the non-braking position, the redundant channelwill apply brakes using the command from the other sensor.

The signal partitioning shown in FIG. 10 also prevents the undesiredasymmetrical release of brakes owing to an ADIRU signal failing to theGroundspeed high mode. A voting scheme was considered that brought allthree ADIRU buses to each digital brake control channel, but this wasrejected because of the high component count and concerns about fan outburden on the ADIRU ARINC 429 transmitters. A second voting schemewhereby the Monitor Card would vote on the ADIRU channels and pass asignal to the brake control channels was rejected because of thepossible of single point failure in the Monitor Card. Asymmetricalbraking can result from unwanted pressure being applied to any brake.Pressure sensors mounted downstream of each servo valve detect thiscondition and the monitor channel select the alternate system thusbypassing the failure.

Referring to FIG. 10, the braking and steering computer has thisfollowing circuit board sub components:

Primary Normal/Alternate

Braking Channel 1—3 cards

Secondary Normal/Alternate

Braking Channel 2—3 cards

BIT/COMM1—1 card

BIT/COMM2—1 card

BITE/Autobrake—1 card

Steering Control—1 card

In the normal braking system six brake control cards provide dualredundant digital brake-by-wire controls. Three cards provide controlfor channel 1 and three cards provide redundant control on channel 2.The BITE/Autobrake card provides normal system BITE (Built-In-TestEquipment) and Autobrake. The normal system BITE card also providesmutually exclusive hydraulic system selection. Communication with thebrake control and steering cards is handled through the BIT/COMM cards.The BIT/COMM cards perform local built-in-test and transfer informationto the BITE card for failure isolation. The Normal/Alternate brakingcards provide alternate braking control. These cards performbrake-by-wire pressure control similar to the normal braking modewithout using software. Alternate control BITE is performed by thenormal system BITE card. The BITE provides extensive test andcommunication capability for the alternate control channel.

A cockpit mounted Autobrake switch panel provides the means for theflight crew to select one of the five landing decelerations or an RTOmode. The five landing deceleration levels are indicated as LO, 2,3,4,and HI. An RTO selector switch provides RTO mode selection to the brakecontrol unit. The Captain or the First Officer will choose adeceleration setting and the brake control system will provide signalsto allow the switch to latch to the selected deceleration position. Theautobrake card will illuminate the Active light when autobrake startspressure application. The autobrake card will illuminate the decel lightwhen the aircraft deceleration is within 80% of the selecteddeceleration. When RTO is selected and armed, the autobrake card willilluminate the RTO Armed light on the autobrake switch panel.

The braking control unit will provide brake control function when theassociated +28 VDC power supply is energized. Brake control consists ofthe following functions as a minimum:

1. Pedal Command

2. Antiskid Protection

3. Touchdown/Aquaplaning Protection (With Spin-up Override

4. Locked Wheel Protection

5. Gear Retract Braking

6. Autobrake Command

7. Pressure Control

A pressure command will be generated from the above functions. Thepressure command for brake control will be the lowest pressure resultingfrom comparing functions 1 through 4. Gear retract braking usessafeguards such as weight-on-wheels (Weight-On-Wheels) inputs, gearposition handle, and gear down and locked switches. During autobrakecontrol the command is the lowest of functions 6, 2, 3, and 4, alongwith all the safeguards of autobrake control.

The braking control unit will provide antiskid protection when theassociated +28 VDC power supply is energized, the antiskid switchindicates antiskid on, and when the wheel speed is above 5.14 M/S (lowspeed dropout velocity). The software will measure the wheel speeds ofall wheels from 123.5 M/S to zero through the axle mounted wheel speedtachometers. The wheel speed information will be used to determineincipient wheel skid conditions and a correction signal will begenerated. This correction signal is the antiskid command. Antiskidcontrol is performed on an individual wheel basis. Antiskid command isone of several inputs to the brake control function. Antiskid for thealternate channel is performed on a paired wheel basis.

The braking and steering control unit will perform closed loop pressurecontrol on all twelve wheels of the normal and alternate systems(alternate system in analog control). The brake control unit willmonitor the brake pressure transducers and use the pressure command andpressure feedback data to perform pressure control. An error signal isgenerated as follows:

PRESSURE ERROR=PRESSURE COMMAND−FEEDBACK PRESSURE

Through a modified PID (Proportional, Integral, Differential) controlloop the software will generate a brake valve command to achieve thecommanded pressure.

The metered pressure of brakes that are below the skid threshold will becontrolled to within 1 bar between a tandem pair of brakes. The systemwill control brake pressure to each pair of brakes to ensure equal load,energy and wear sharing during taxi near the brake applicationthreshold.

Touchdown and aquaplaning protection is provided by comparing wheelspeeds with the groundspeed signal from the Air Data Inertial ReferenceUnits (ADIRUs). Any main gear aft wheel that is at a velocity 50 knotsor more below the ADIRU groundspeed value is given a brake releasesignal. On the center gear, the left aft and right forward wheels areprotected. The remaining main gear and center gear wheels are protectedby the locked wheel protection feature. Touchdown and aquaplaningprotection is inhibited once the groundspeed falls below 50 kts. TheADIRU signals are partitioned as shown in FIG. 9 so that redundantchannels gets the same ADIRU.

In the event that an ADIRU channel falls to the high speed mode, thenormal controllers using the Signal will release two of their associatedbrakes. If two ADIRU channels both fail to the high speed mode a maximumof four brakes will be released. In the event that an ADIRU channel isunavailable or falls to the low speed mode, aquaplaning and touchdownprotection would be lost to wheels on the affected channels. The loss ofone ADIRU would cause loss of direct protection to two wheels and lossof indirect protection to four wheels in a symmetrical fashion. Thisprobability can be diminished by using the Weight-On-Wheels (WOW) signin combination with a spin up override signal. Using this approach thesystem will inhibit pressure to the brakes for 3 seconds tuneable) afterground mode has been established or until the wheels have spun up,whichever occurs first.

Locked wheel protection will command zero pressure (system returnpressure) to the brakes when a locked wheel condition is determined.Locked wheel protection compares the wheel speed of a wheel with that ofits partner wheel. If the wheel speed of a wheel is 30% or less(tuneable) of its partner then a locked wheel condition is declared andzero pressure will be commanded to the locked brake. The factor of 30%has been chosen to allow for a flat tire running on the rim. Lockedwheel crossover protection command is one of several inputs to the brakecontrol function.

LOCKED WHEEL PAIRING: Fore-aft wheels are paired together as follows:

Wheel 1 pair Wheel 5 Wheel 2 pair Wheel 6 Wheel 9 pair Wheel 11 Wheel 10pair Wheel 12 Wheel 3 pair Wheel 7 Wheel 4 pair Wheel 8

The brake control system will initiate gear retract braking when thelanding gear lever is placed in the not down position after theweight-on-wheels signal has transitioned from ground mode to air modeand the main gear is not down and locked. A predetermined pressure willbe applied to all brakes for three seconds or until the nose landinggear is no longer down and locked. The main gear down and locked inputis included for additional safety and if deemed unnecessary may beremoved.

The brake control card will receive the autobrake command from theBITE/Autobrake card. This command will be validated and used forautobrake control. The brake control card will monitor and evaluateautobraking engagement through the spoiler deployment indications, andthe autobrake arming status. If the pedals are depressed beyond apredefined level at any time that autobrake is engaged/armed, the brakecontrol card will disregard the autobrake command and assume normalantiskid braking effort.

The braking control unit will provide provisions for autobrake controlwith five different deceleration selection settings, and an RTOselection mode. The deceleration settings are tuneable and defined as:

Settings: OFF

LO

2

3

4

HI

and an RTO selection switch.

The following functions are performed in order to initiate autobrakecontrol:

1. Autobrake Arming

2. Autobrake Deceleration Error Calculation

3. Autobrake Command Calculation

4. Autobrake Switch Panel Indication Control

The proposed Autobrake control uses deceleration and ground speedreceived from the IRS (Inertial Reference System) via the ARINC 429 databus. Data from the three independent ADIRU channels will be compared forvalidity and agreement in order to discard incorrect data.

The BITE/Autobrake card will read the setting of the autobrake switchand check the aircraft discretes for arming conditions. The followingconditions will be met as a minimum for arming the Autobrake:

1. Autobrake switch at (LO, 2, 3, 4, HI, or RTO)

2. Primary hydraulic system pressurized

3. No external or Brake System failure that would prevent operation ofnormal braking on all braked wheels (TBC)

4. FCPC's available (at least 2)

5. ADIRU's available

When all of the above minimum arming conditions have been satisfied,then the BSCU will provide the appropriate signals to the autobrakeswitch panel to latch the switch in position and transmit the discrete‘ARM′’ bits in ARINC 429 form for each autobrake mode.

The BITE/Autobrake card will disarm the autobrake and remove theautobrake switch latching conditions (including the ‘ARM’ discrete toARINC) when any of the following conditions are encountered:

1. The selected autobrake mode is deselected

2. Any arming condition is lost

3. The aircraft is in ‘flight’ for more than 10 Sec.

4. One or both brake pedals are depressed past a threshold (TBD) (forlanding autobrake modes only)

In the above cases the braking control unit will control the rotaryswitch to the OFF position, or if in the RTO mode remove the ‘ARM’caption on the display. In the case of item 2, the BSCU will transmit an“AUTOBRAKE FAULT’ output on ARINC 429. In the case of items 1, 3, and 4the BSCU will transmit an ‘AUTOBRAKE OFF’ output on ARINC 429.

When all of the arming conditions are satisfied the BSCU will initiateautobraking on the Normal Braking channel when the following conditionsare met:

Landing mode:

Two out of three ground spoiler deployment Signals present, the NoseLanding Gear is on the ground (TBC) and a delay time has expired (TBD).

RTO mode:

Two out of three ground spoiler deployment signals present (no delay).

On engagement the BSCU will control the “ACTIVE” caption on the display.Pressure deployment will be smooth and controlled. A brake fill pressurespike will precede the autobrake pressure ramp-up to remove anyunnecessary application delays and to remove unnecessary torque spikesof initial application When a new deceleration is selected the pressurechange rate will be smooth and controlled in such a manner as tomaximize passenger comfort.

Autobrake will disengage when any of the following conditions occur:

1. Any arming condition disappears

2. During operation two out of three ground spoiler signals are nolonger present

3. During operation one or both brake pedals are depressed.

In the case of 1 disarming would operate as discussed above. In the caseof 2 the ‘ACTIVE’ caption will disappear. In the case of 3 the BSCU willcontrol the autobrake switch to the OFF position and the ‘ARM’ captionwill disappear from the display panel. In the case of 2 and 3 the BSCUwill transmit an ‘AUTOBRAKE OFF’ output on ARINC 429.

The braking and steering control unit autobrake will provide the highestdegree of performance possible and will minimize the effects of thenatural frequencies of the gear. The autobrake will control the overalldeceleration of the aircraft. Other aircraft retarding effects such asuse of the ground spoilers or thrust reversers will not affect theoverall aircraft deceleration if the contributions from these factorsare less than the selected deceleration value. During engagement anddisengagement of the landing mode, including pedal brake take-over,pressure transitions will be smooth and less than 5 bars. Once theselected deceleration has been achieved, the average deceleration willnot vary from the desired deceleration by more than 3%. Provided thereis sufficient runway friction to obtain the desired deceleration,transient variations in deceleration will be less than ±10% of thereference deceleration for all aircraft speeds and configurations. Sincethe brake control algorithms are used for autobrake control, equal loadsharing is guaranteed and temperature variations among brakes will bekept to a minimum.

Autobrake command calculation and update rate will be 20 milliseconds asa minimum. Data from the three ADIRU's are validated and used for thecalculation of the autobrake command.

The BSCU BITE has been structured to use three independent cards. TwoBIT/COMM cards (for Primary and Secondary channels) providecommunication and local built in test for each channel. The BIT/COMMcards also provide the CMC, SPATIAL, and BSCU output ARINC 429interfaces. A master BITE/Autobrake card provides fault isolation forthe whole BSCU. The BITE/Autobrake card provides independent monitoringfor the steering control card. The BITE/Autobrake card serves as acommunication master and initiates all communication to the BIT/COMMcards. Each BIT/COMM card works with data from its associated threebrake control cards. The BIT/COMM cards perform local fault isolationand transmits results to the BITE/Autobrake card. Each brake control andthe steering control card performs it's own fault monitoring andtransmits data to the BIT/COMM cards.

The brake control unit contains six brake control cards. Each BSCU brakecontrol channel consists of three brake control cards. Primary andsecondary brake control cards operate simultaneously to provideredundancy to the brake control function. The brake control card isdesigned to contain all the elements needed to control four wheels inthe primary and alternate mode. The brake control card will use a highperformance microprocessor. The following functions will be included asa minimum:

1. Pedal Interface

2. Pressure Transducer Interface

3. Wheelspeed Transducer Interface

4. Discrete Input Interface

5. Shutoff Valve Interface

6. Brake Control Valve Interface

7. External Velocity Interface

8. Asynchronous Serial Interface

9. Low Power Reset

10. Hardware Watchdog

11. Power Supply

12. Microprocessor related hardware (memory, I/O, latch, etc.)

The brake control card hardware will interface with four pedalpotentiometers (Captain, First Officer, left and right pedals). Thepedal interface will provide analog output to the brake control cardanalog inputs (A/D converter inputs). The brake control card hardwarewill interface with six pressure transducers (Normal, and Alternate).The pressure transducer interface hardware produces analog output linearto input pressure. The pressure transducer interface hardware output ismultiplexed prior to input to the microcontroller analog input channels.The brake control card will provide the +15 VDC excitation to thepressure transducers. A test signal will be provided for each pressuretransducer. This test signal is checked for out of tolerance limits andis multiplexed prior to input to the analog inputs.

The brake control card hardware will interface with four wheel speedtransducers. The hardware will provide the appropriate wheel speedtransducer oil bias current (typically 10 ma). The hardware will providea hysteresis voltage in the detection circuitry to avoid noisetriggering a wheel speed output. The hardware will convert the wheelspeed interface sine wave signal to pulses proportional to wheelrotational velocity. These pulses are processed by the computer toderive wheel speed. A Schmitt Trigger interface is used to avoid falsetriggering.

The brake control unit will interface with various aircraft discretesThe following list defines the minimum number of discrete input sources:

1. BSCU Pin Programming (if Needed)

2. Primary Hydraulic System Low Pressure

3. Secondary Hydraulic System Low Pressure

4. Landing Gear Selector Lever UP

5. Landing Gear Down and Locked

6. Flight/Ground switches

7. Autobrake Arming Demands (Autobrake Switch)

8. Antiskid and Nose Wheel Steering OFF Switch

9. Engine Master Lever Position

10. Fans On

11. SPATIAL Activation

Each brake control card will be able to supply drive current to sixvalves (normal and alternate servo valves). Each valve driver will havecapability to output 15 mA minimum per valve. The brake control cardwill have capability to detect open circuit/short circuit valve. Thevalve voltage is scaled and multiplexed into the analog to digitalconverter.

An independent current source (external to the brake control valvedriver) will interface to the brake control valves. The additionalcurrent source provides means of fault isolation of the brake controlvalve driver interfaced. Additional initiated tests are performed suchas resistive measurements of the wheel speed transducer utilizing bothcurrent sources. The external current source pulsing is through softwarecontrol. This test is performed only when it is determined that is safeto do so, i.e. wheel speed=0 and velocity reference=0.

The brake control hardware will provide provisions for interfacing withthe BIT/COMM cards. The serial link will provide high speed (TBD KBAUD)transmit and receive links to the BIT/COMM cards.

The brake control hardware will provide provisions for power-up and lowpower reset. Power up reset function holds the computer in resetcondition until the power supplies have settled to the proper powerlevels (i.e. 5 volt digital, 5 volt analog reference). The reset timewill be adjusted to achieve proper reset opera Low power reset issimilar to power up reset. The low power reset circuitry will reset thecomputer when input power drops below the regulating limits of the 5volts power supply.

The two hydraulic accumulators provide braking to the twelve wheels, asshown in the simplified schematic in FIG. 11, after 18 hours of flight,followed by seven brake applications to brake pressure of up to 100bars. In this cast the Parking/Ultimate valve may leak at its maximumspecified leakage rate of 6.0 cc/hr for the entire flight duration,which reduces the accumulator pressure to 201 bar, a remaining fluidvolume of 7523 cm³.

After the depletion phase described above, an initial brake applicationwill consume 989 cm³ followed by additional six brake applications frombrake contact pressure of 21 bar to brake pressure of 100 bar. After theseventh brake application the accumulators pressure is 102 bar with aremaining hydraulic fluid volume of 4721 cm³.

It will be apparent from the foregoing that while particular forms ofthe invention have been illustrated and described, various modificationscan be made without departing from the spirit and scope of theinvention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

What is claimed is:
 1. In combination with an aircraft, an apparatus fordual redundant control of hydraulically operated wheel braking for theaircraft, the aircraft having right and left main landing gear andcenter landing gear that can move between a retracted position and anactuated position, the landing gear having a plurality of wheels and acorresponding plurality of wheel brakes for the plurality of wheels, anda plurality of brake pedals for controlling operation of braking of saidplurality of said wheels, said apparatus comprising: a primary hydraulicsystem connected in fluid communication with said plurality of wheelbrakes for providing hydraulic power for normal operation of saidplurality of wheel brakes in a normal braking mode, said primaryhydraulic system comprising a plurality of primary hydraulic fluidcontrol channels and a corresponding plurality of secondary hydraulicfluid control channels, the primary and secondary hydraulic fluidcontrol channels operating simultaneously and autonomously, and beingredundant and partitioned among said plurality of wheel brakes so thateven if one of the plurality of primary hydraulic fluid- controlchannels and a corresponding one of the plurality of secondary hydraulicfluid control channels fail to apply pressure, braking will be lost toonly a portion of the wheel brakes and the loss will be in a symmetricalpattern; a secondary hydraulic system connected in fluid communicationwith said plurality of wheel brakes for providing hydraulic power foroperation of said plurality of wheel brakes in an alternate brakingmode, said secondary hydraulic system providing dual redundant analogbrake-by-wire control in the alternate braking mode for the right andleft main and center landing gears of the aircraft, and said secondaryhydraulic system comprising at least one primary hydraulic fluid controlchannel and at least one secondary hydraulic fluid control channel; acontrol unit for controlling brake pressure communicated to said wheelbrakes through said primary and secondary hydraulic systems; and amonitor channel operatively connected to said primary and secondaryhydraulic systems for detecting faults in said primary and secondaryhydraulic systems and for selecting between the primary and secondaryhydraulic systems for providing braking pressure.
 2. The apparatus ofclaim 1, wherein said plurality of wheel brakes comprises a front pairof wheel brakes and an aft pair of wheel brakes for each of said rightand left main landing gear, and wherein wheel braking power is providedby main landing gear common fluid channels to adjacent ones of saidfront and aft wheel brakes to provide protection against asymmetricalwheel braking.
 3. The apparatus of claim 1, wherein said plurality ofbrake pedals comprises a plurality of left brake pedals and a pluralityof right brake pedals for controlling braking, and wherein saidplurality of wheel brakes comprises a front pair of wheel brakes and anaft pair of wheel brakes, and wherein said primary and secondaryhydraulic fluid control channels control all four wheels of the centerlanding gear.
 4. The apparatus of claim 3, wherein wheel braking poweris provided by center landing gear common fluid channels to the wheelbrakes of the center landing gear on an axle pair basis.
 5. Theapparatus of claim 1, wherein said plurality of wheel brakes aresymmetrically divided into three sets of wheel brakes, and wherein saidprimary hydraulic system comprises first, second and third redundantpairs of primary and secondary fluid control channels, said firstredundant pair of primary and secondary fluid control channels includinga first primary hydraulic fluid control channel and a first secondaryhydraulic fluid control channel, each of said first primary and saidfirst secondary hydraulic fluid control channels controlling said firstset of wheel brakes, said second redundant pair of primary and secondaryfluid control channels including a second primary hydraulic fluidcontrol channel and a second secondary hydraulic fluid control channel,each of said second primary and said second secondary hydraulic fluidcontrol channels controlling said second set of wheel brakes, said thirdredundant pair of primary and secondary fluid control channels includinga third primary hydraulic fluid control channel and a third secondaryhydraulic fluid control channel, each of said third primary and saidthird secondary hydraulic fluid control channels controlling said thirdset of wheel brakes.
 6. The apparatus of claim 5, wherein each said pairof primary and secondary hydraulic fluid control channels controls eachof said night and left main landing gear and center landing gear.
 7. Theapparatus of claim 1, wherein said secondary hydraulic system providespressure for alternate braking using dual, independent, closed loopanalog control circuits.
 8. The apparatus of claim 1, wherein saidsecondary hydraulic system comprises a plurality of accumulators forproviding an alternate supply of hydraulic power.
 9. The apparatus ofclaim 8, wherein said alternate supply of hydraulic power is providedfor an emergency braking mode in the event that both the primaryhydraulic system and the secondary hydraulic system are depressurized.10. The apparatus of claim 8, wherein said alternate supply of hydraulicpower is provided for an ultimate braking mode providing brakingpressure to said plurality of wheel brakes.
 11. The apparatus of claim8, wherein said alternate supply of hydraulic power is provided for aparking brake mode providing braking pressure to said plurality of wheelbrakes.
 12. The apparatus of claim 1, wherein said monitor channeldetects occurrence of loss of pressure in the primary hydraulic system,if any brake has unwanted pressure applied, and if a fault is detectedon the primary or secondary hydraulic systems that affects more than onewheel brake on each landing gear.
 13. The apparatus of claim 1, furthercomprising first and second solenoid operated shut-off valvesoperatively connected to said primary and secondary hydraulic systems,respectively, and to said control unit for selecting operation of one ofsaid primary and secondary hydraulic systems, said first and secondsolenoid operated shut-off valves being configured to operate in amutually exclusive manner to positively select between operation of saidprimary and secondary hydraulic systems without the possibility ofhaving both systems pressurized at the same time.
 14. The apparatus ofclaim 13, wherein said control unit comprises thrust levers, and saidsolenoid operated shut-off valves are implemented through said thrustlevers to positively prevent pressure from the primary hydraulic systemand secondary hydraulic system being applied to the primary or secondaryhydraulic systems during take off.
 15. In combination with an aircraft,an apparatus for dual redundant control of hydraulically operated wheelbraking for the aircraft, the aircraft having right and left mainlanding gear and center landing gear that can move between a retractedposition and an actuated position, the landing gear having a pluralityof wheels and a corresponding plurality of wheel brakes- for theplurality of wheels, and a plurality of brake pedals for controllingoperation of braking of said plurality of said wheels, said apparatuscomprising: a primary hydraulic system connected in fluid communicationwith said plurality of wheel brakes in a normal braking mode, saidprimary hydraulic system comprising a plurality of primary hydraulicfluid control channels and a plurality of secondary hydraulic fluidcontrol channels, the primary and secondary hydraulic fluid controlchannels operating simultaneously and autonomously, and being redundantand partitioned among said plurality of wheel brakes so that even ifboth the primary and secondary hydraulic fluid control channels fail toapply pressure, braking will be lost to only a portion of the wheelbrakes and the loss will be in a symmetrical pattern; a secondaryhydraulic system connected in fluid communication with said plurality ofwheel brakes for providing hydraulic power for operation of saidplurality of wheel brakes in an alternate braking mode, said secondaryhydraulic system providing dual redundant analog brake-by-wire controlin the alternate braking mode for the right and left main and centerlanding gears of the aircraft, and said secondary hydraulic systemcomprising at least one primary hydraulic fluid control channel and atleast one secondary hydraulic fluid control channel; a control unit forcontrolling brake pressure communicated to said wheel brakes throughsaid primary and secondary hydraulic systems; a monitor channeloperatively connected to said primary and secondary hydraulic systemsfor detecting faults in said primary and secondary hydraulic systems andfor selecting between the primary and secondary hydraulic systems forproviding braking pressure; first and second solenoid operated shut-offvalves operatively connected to said primary and secondary hydraulicsystems, respectively, and to said control unit for selecting operationof one of said primary and secondary hydraulic systems, said first andsecond solenoid operated shut-off valves being configured to operate ina mutually exclusive manner to positively select between operation ofsaid primary and secondary hydraulic systems without the possibility ofhaving both systems pressurized at the same time, wherein said controlunit comprises thrust levers, and said solenoid operated shut-off valvesare implemented through said thrust levers to positively preventpressure from the primary hydraulic system and secondary hydraulicsystem being applied to the primary or secondary hydraulic systemsduring take off; and a landing gear lever controlling retraction of thelanding gear, and dual redundant switches on the landing gear lever tobypass the thrust lever switches to stop the wheels during climb whenthe thrust levers are advanced to enable wheel braking upon retractionof the landing gear.
 16. The apparatus of claim 15, wherein the centerlanding gear comprises a nose landing gear, and each of the primarybrake hydraulic fluid control channels receives a software independentsignal that initiates retraction braking for three seconds or until thenose landing gear is up and locked, whichever happens sooner.
 17. Theapparatus of claim 15, wherein an anti-skid function of the normalbraking mode is inhibited during retraction braking.
 18. The apparatusof claim 14, wherein said control unit comprises a plurality of servocontrol valves controlled by corresponding dual solenoid coils forcontrolling the operation of said wheel brakes, respectively, andwherein said thrust levers comprise dual thrust lever switches thatbreak both power and ground to said first and second solenoid operatedshut-off valves for the primary hydraulic system and secondary hydraulicsystems when one of said thrust levers is advanced.
 19. The apparatus ofclaim 14, wherein said control unit further comprises a plurality ofsensors for sensing the position of each said brake pedal.
 20. Incombination with an aircraft, an apparatus for dual redundant control ofhydraulically operated wheel braking for the aircraft, the aircrafthaving right and left main landing gear and center landing gear that canmove between a retracted position and an actuated position, the landinggear having a plurality of wheels and a corresponding plurality of wheelbrakes for the plurality of wheels, and a plurality of brake pedals forcontrolling operation of braking of said plurality of said wheels, saidapparatus comprising: a primary hydraulic system connected in fluidcommunication with said plurality of wheel brakes for providinghydraulic power for normal operation of said plurality of wheel brakesin a normal braking mode, said primary hydraulic system comprising aplurality of primary hydraulic fluid control channels and a plurality ofsecondary hydraulic fluid control channels, the primary-and secondaryhydraulic fluid control channels operating simultaneously andautonomously, and being redundant and partitioned among said pluralityof wheel brakes so that even if both the primary and secondary hydraulicfluid control channels fail to apply pressure, braking will be lost toonly a portion of the wheel brakes and the loss will be in a symmetricalpattern; a secondary hydraulic system connected in fluid communicationwith said plurality of wheel brakes for providing hydraulic power foroperation of said plurality of wheel brakes in an alternate brakingmode, said secondary hydraulic system providing dual redundant analogbrake-by-wire control in the alternate braking mode for the right andleft main and center landing gears of the aircraft, and said secondaryhydraulic system comprising at least one primary hydraulic fluid controlchannel and at least one secondary hydraulic fluid control channel; acontrol unit for controlling brake pressure communicated to said wheelbrakes through said primary and secondary hydraulic systems; a monitorchannel operatively connected to said primary and secondary hydraulicsystems for detecting faults in said primary and secondary hydraulicsystems and for selecting between the primary and secondary hydraulicsystems for providing braking pressure; first and second solenoidoperated shut-off valves operatively connected to said primary andsecondary hydraulic systems, respectively, and to said control unit forselecting operation of one of said primary and secondary hydraulicsystems, said first and second solenoid operated shut-off valves beingconfigured to operate in a mutually exclusive manner to positivelyselect between operation of said primary and secondary hydraulic systemswithout the possibility of having both systems pressurized at the sametime, wherein said control unit comprises thrust levers, and saidsolenoid operated shut-off valves are implemented through said thrustlevers to positively prevent pressure from the primary hydraulic systemand secondary hydraulic system being applied to the primary or secondaryhydraulic systems during take off; wherein said control unit furthercomprises a plurality of sensors for sensing the position of each saidbrake pedal, and wherein said plurality of sensors comprises dualredundant switches that break both power and ground to said first andsecond solenoid operated shut-off valves, such that depression of one ofsaid brake pedals opens the first and second solenoid operated shut-offvalve for one of the primary hydraulic system in the normal braking modeand the secondary hydraulic system in the alternate braking mode.
 21. Incombination with an aircraft, an apparatus for dual redundant control ofhydraulically operated wheel braking for the aircraft, the aircrafthaving right and left main landing gear and center landing gear that canmove between a retracted position and an actuated position, the landinggear having a plurality of wheels and a corresponding plurality of wheelbrakes for the plurality of wheels, and a plurality of brake pedals forcontrolling operation of braking of said plurality of said wheels, saidapparatus comprising: a primary hydraulic system connected in fluidcommunication with said plurality of wheel brakes for providinghydraulic power for normal operation of said plurality of wheel brakesin a normal braking mode, said primary hydraulic system comprising aplurality of primary hydraulic fluid control channels and a plurality ofsecondary hydraulic fluid control channels, the primary and secondaryhydraulic fluid control channels operating simultaneously andautonomously, and being redundant and partitioned among said pluralityof wheel brakes so that even if both the primary and secondary hydraulicfluid control channels fail to apply pressure, braking will be lost toonly a portion of the wheel brakes and the loss will be in a symmetricalpattern; a secondary hydraulic system connected in fluid communicationwith said plurality of wheel brakes for providing hydraulic power foroperation of said plurality of wheel brakes in an alternate brakingmode, said secondary hydraulic system providing dual redundant analogbrake-by-wire control in the alternate braking mode for the right andleft main and center landing gears of the aircraft, and said secondaryhydraulic system comprising at least one primary hydraulic fluid controlchannel and at least one secondary hydraulic fluid control channel; acontrol unit for controlling brake pressure communicated to said wheelbrakes through said primary and secondary hydraulic systems; a monitorchannel operatively connected to said primary and secondary hydraulicsystems for detecting faults in said primary and secondary hydraulicsystems and for selecting between the primary and secondary hydraulicsystems for providing braking pressure; and first and second solenoidoperated shut-off valves operatively connected to said primary andsecondary hydraulic systems, respectively, and to said control unit forselecting operation of one of said primary and secondary hydraulicsystems, said first and second solenoid operated shut-off valves beingconfigured to operate in a mutually exclusive manner to positivelyselect between operation of said primary and secondary hydraulic systemswithout the possibility of having both systems pressurized at the sametime, wherein said control unit comprises thrust levers, and saidsolenoid operated shut-off valves are implemented through said thrustlevers to positively prevent pressure from the primary hydraulic systemand secondary hydraulic system being applied to the primary or secondaryhydraulic systems during take off; wherein said first and secondsolenoid operated shut-off valves also turn off hydraulic power to saidservo control valves during flight.
 22. The apparatus of claim 14,wherein said first and second solenoid operated shut-off valves comprisethree way solenoid operated shut-off valves.
 23. The apparatus of claim14, wherein said control unit further comprises sensor means fordetermining brake pedal application and for generating a pedalapplication signal indicating when one of said brake pedals has beenapplied.
 24. In combination with an aircraft, an apparatus for dualredundant control of hydraulically operated wheel braking for theaircraft, the aircraft having right and left main landing gear andcenter landing gear that can move between a retracted position and anactuated position, the landing gear having a plurality of wheels and acorresponding plurality of wheel brakes for the plurality of wheels, anda plurality of brake pedals for controlling operation of braking of saidplurality of said wheels, said apparatus comprising: a primary hydraulicsystem connected in fluid communication with said plurality of wheelbrakes for providing hydraulic power for normal operation of saidplurality of wheel brakes in a normal braking mode, said primaryhydraulic system comprising a plurality of primary hydraulic fluidcontrol channels and a plurality of secondary hydraulic fluid controlchannels, the primary and secondary hydraulic fluid control channelsoperating simultaneously and autonomously, and being redundant andpartitioned among said plurality of wheel brakes so that even if boththe primary and secondary hydraulic fluid control channels fail to applypressure, braking will be lost to only a portion of the wheel brakes andthe loss will be in a symmetrical pattern; a secondary hydraulic systemconnected in fluid communication with said plurality of wheel brakes forproviding hydraulic power for operation of said plurality of wheelbrakes in an alternate braking mode, said secondary hydraulic systemproviding dual redundant analog brake-by-wire control in the alternatebraking mode for the right and left main and center landing gears of theaircraft, and said secondary hydraulic system comprising at least oneprimary hydraulic fluid control channel and at least one secondaryhydraulic fluid control channel; a control unit for controlling brakepressure communicated to said wheel brakes through said primary andsecondary hydraulic systems; a monitor channel operatively connected tosaid primary and secondary hydraulic systems for detecting faults insaid primary and secondary hydraulic systems and for selecting betweenthe primary and secondary hydraulic systems for providing brakingpressure; and first and second solenoid operated shut-off valvesoperatively connected to said primary and secondary hydraulic systems,respectively, and to said control unit for selecting operation of one ofsaid primary and secondary hydraulic systems, said first and secondsolenoid operated shut-off valves being configured to operate in amutually exclusive manner to positively select between operation of saidprimary and secondary hydraulic systems without the possibility ofhaving both systems pressurized at the same time, wherein said controlunit comprises thrust levers, and said solenoid operated shut-off valvesare implemented through said thrust levers to positively preventpressure from the primary hydraulic system and secondary hydraulicsystem being applied to the primary or secondary hydraulic systemsduring take off; wherein said control unit further comprises means forsensing weight on said plurality of wheels and for generating a brakeinhibit signal when weight is not applied on said plurality of wheels.25. The apparatus of claim 1, wherein said control unit furthercomprises a plurality of servo control valves for controlling theoperation of said wheel brakes.
 26. The apparatus of claim 25, whereinsaid plurality of servo control valves are controlled by correspondingdual solenoid coils.
 27. The apparatus of claim 25, wherein saidplurality of servo control valves are further controlled with pressurefeedback.
 28. The apparatus of claim 25, wherein said plurality of servocontrol valves comprises twelve dual coil brake control valves in theprimary hydraulic fluid system.
 29. The apparatus of claim 27, furthercomprising antiskid control provided on each of said plurality of wheelbrakes utilizing the servo control valves with pressure feedback. 30.The apparatus of claim 1, wherein said control unit comprises system sixbrake control cards providing dual redundant digital brake-by-wirecontrols.
 31. The apparatus of claim 1, wherein said control unitcomprises a built-in-test equipment card for said primary hydraulicfluid system providing mutually exclusive hydraulic system selection.32. The apparatus of claim 25, wherein said control unit comprises apressure sensor mounted downstream of each servo control valve fordetecting asymmetrical braking due to unwanted pressure applied to anyof said plurality of wheel brakes, whereby the monitor channel canselect the alternate hydraulic fluid system.
 33. The apparatus of claim1, further comprising an autobrake control subsystem.
 34. The apparatusof claim 3, wherein said plurality of brake pedals comprises a pluralityof left brake pedals and a plurality of right brake pedals, and saidprimary and secondary hydraulic fluid control channels are connected tosaid front and aft pairs of wheel brakes such that the left brake pedalscontrol the front pair and the right brake pedals control the aft pair.35. The apparatus of claim 9, wherein said plurality of wheel brakescomprises center landing gear and main landing gear wheel brakes, and inthe emergency braking mode said plurality of accumulators provideshydraulic power to said center landing gear and main landing gear wheelbrakes.
 36. In combination with an aircraft, an apparatus for dualredundant control of hydraulically operated wheel braking for theaircraft, the aircraft having landing gear that can move between aretracted position and an actuated position, the landing gear having aplurality of wheels and a corresponding plurality of wheel brakes forthe plurality of wheels, wherein said plurality of wheel brakes aresymmetrically divided into three sets of wheel brakes, and a pluralityof brake pedals for controlling operation of braking of said pluralityof said wheels, said apparatus comprising: a primary hydraulic systemconnected in fluid communication with said plurality of wheel brakes forproviding hydraulic power for normal operation of said plurality ofwheel brakes in a normal braking mode, said primary hydraulic systemcomprising a plurality of primary hydraulic fluid control channels and aplurality of secondary hydraulic fluid control channels, the primary andsecondary hydraulic fluid control channels operating simultaneously andautonomously, and wherein said primary hydraulic system comprises first,second and third redundant pairs of primary and secondary fluid controlchannels, said first redundant pair of primary and secondary fluidcontrol channels including a first primary hydraulic fluid controlchannel and a first secondary hydraulic fluid control channel, each ofsaid first primary and said first secondary hydraulic fluid controlchannels controlling said first set of wheel brakes, said secondredundant pair of primary and secondary fluid control channels includinga second primary hydraulic fluid control channel and a second secondaryhydraulic fluid control channel, each of said second primary and saidsecond secondary hydraulic fluid control channels controlling saidsecond set of wheel brakes, said third redundant pair of primary andsecondary fluid control channels including a third primary hydraulicfluid control channel and a third secondary hydraulic fluid controlchannel, each of said third primary and said third secondary hydraulicfluid control channels controlling said third set of wheel brakes; asecondary hydraulic system connected in fluid communication with saidplurality of wheel brakes for providing hydraulic power for operation ofsaid plurality of wheel brakes in an alternate braking mode, saidsecondary hydraulic system comprising at least one primary hydraulicfluid control channel and at least one secondary hydraulic fluid controlchannel; a control unit for controlling brake pressure communicated tosaid wheel brakes through said primary and secondary hydraulic systems;a monitor channel operatively connected to said primary and secondaryhydraulic systems for detecting faults in said primary and secondaryhydraulic systems and for selecting between the primary and secondaryhydraulic systems for providing braking pressure; and first and secondsolenoid operated shut-off valves operatively connected to said primaryand secondary hydraulic systems, respectively, and to said control unitfor selecting operation of one of said primary and secondary hydraulicsystems, said first and second solenoid operated shut-off valves beingconfigured to operate in a mutually exclusive manner to positivelyselect between operation of said primary and secondary hydraulic systemswithout the possibility of having both systems pressurized at the sametime, wherein said control unit comprises thrust levers, and saidsolenoid operated shut- off valves are implemented through said thrustlevers to positively prevent pressure from the primary hydraulic systemand secondary hydraulic system being applied to the primary or secondaryhydraulic systems during take off.
 37. In combination with an aircraft,an apparatus for dual redundant control of hydraulically operated wheelbraking for the aircraft, the aircraft having landing gear that can movebetween a retracted position and an actuated position, the landing gearhaving a plurality of wheels and a corresponding plurality of wheelbrakes for the plurality of wheels, wherein said plurality of wheelbrakes are symmetrically divided into three sets of wheel brakes, and aplurality of brake pedals for controlling operation of braking of saidplurality of said wheels, said apparatus comprising: a primary hydraulicsystem connected in fluid communication with said plurality of wheelbrakes for providing hydraulic power for normal operation of saidplurality of wheel brakes in a normal braking mode, said primaryhydraulic system comprising at least one primary hydraulic fluid controlchannel and at least one secondary hydraulic fluid control channel, theprimary and secondary hydraulic fluid control channels operatingsimultaneously and autonomously, and wherein said primary hydraulicsystem comprises first, second and third redundant pairs of primary andsecondary fluid control channels, said first redundant pair of primaryand secondary fluid control channels including a first primary hydraulicfluid control channel and a first secondary hydraulic fluid controlchannel, each of said first primary and said first secondary hydraulicfluid control channels controlling said first set of wheel brakes, saidsecond redundant pair of primary and secondary fluid control channelsincluding a second primary hydraulic fluid control channel and a secondsecondary hydraulic fluid control channel, each of said second primaryand said second secondary hydraulic fluid control channels controllingsaid second set of wheel brakes, said third redundant pair of primaryand secondary fluid control channels including a third primary hydraulicfluid control channel and a third secondary hydraulic fluid controlchannel, each of said third primary and said third secondary hydraulicfluid control channels controlling said third set of wheel brakes; asecondary hydraulic system connected in fluid communication with saidplurality of wheel brakes for providing hydraulic power for operation ofsaid plurality of wheel brakes in an alternate braking mode, saidsecondary hydraulic system comprising at least one primary hydraulicfluid control channel and at least one secondary hydraulic fluid controlchannel; a control unit for controlling brake pressure communicated tosaid wheel brakes through said primary and secondary hydraulic systems;a monitor channel operatively connected to said primary and secondaryhydraulic systems for detecting faults in said primary and secondaryhydraulic systems and for selecting between the primary and secondaryhydraulic systems for providing braking pressure; first and secondsolenoid operated shut-off valves operatively connected to said primaryand secondary hydraulic systems, respectively, and to said control unitfor selecting operation of one of said primary and secondary hydraulicsystems, said first and second solenoid operated shut-off valves beingconfigured to operate in a mutually exclusive manner to positivelyselect between operation of said primary and secondary hydraulic systemswithout the possibility of having both systems pressurized at the sametime, wherein said control unit comprises thrust levers, and saidsolenoid operated shut-off valves are implemented through said thrustlevers to positively prevent pressure from the primary hydraulic systemand secondary hydraulic system being applied to the primary or secondaryhydraulic systems during take off; and a landing gear lever controllingretraction of the landing gear, and dual redundant switches on thelanding gear lever to bypass the thrust lever switches to stop thewheels during climb when the thrust levers are advanced to enable wheelbraking upon retraction of the landing gear.
 38. The apparatus of claim37, wherein the center landing gear comprises a nose landing gear, andeach of the primary brake hydraulic fluid control channels receives asoftware independent signal that initiates retraction braking for threeseconds or until the nose landing gear is up and locked, whicheverhappens sooner.
 39. The apparatus of claim 37, wherein an anti-skidfunction of the normal braking mode is inhibited during retractionbraking.
 40. The apparatus of claim 36, wherein said control unitcomprises a plurality of servo control valves controlled bycorresponding dual solenoid coils for controlling the operation of saidwheel brakes, respectively, and wherein said thrust levers comprise dualthrust lever switches that break both power and ground to said first andsecond solenoid operated shut-off valves for the primary hydraulicsystem and secondary hydraulic systems when one of said thrust levers isadvanced.
 41. The apparatus of claim 38, wherein said control unitfurther comprises a plurality of sensors for sensing the position ofeach said brake pedal.
 42. The apparatus of claim 41, wherein saidplurality of sensors comprises dual redundant switches that break bothpower and ground to said first and second solenoid operated shut-offvalves, such that depression of one of said brake pedals opens the firstand second solenoid operated shut-off valve for one of the primaryhydraulic system in the normal braking mode and the secondary hydraulicsystem in the alternate braking mode.
 43. In combination with anaircraft, an apparatus for dual redundant control of hydraulicallyoperated wheel braking for the aircraft, the aircraft having landinggear that can move between a retracted position and an actuatedposition, the landing gear having a plurality of wheels and acorresponding plurality of wheel brakes for the plurality of wheels,wherein said plurality of wheel brakes are symmetrically divided intothree sets of wheel brakes, and a plurality of brake pedals forcontrolling operation of braking of said plurality of said wheels, saidapparatus comprising: a primary hydraulic system connected in fluidcommunication with said plurality of wheel brakes for providinghydraulic power for normal operation of said plurality of wheel brakesin a normal braking mode, said primary hydraulic system comprising atleast one primary hydraulic fluid control channel and at least onesecondary hydraulic fluid control channel, the primary and secondaryhydraulic fluid control channels operating simultaneously andautonomously, and wherein said primary hydraulic system comprises first,second and third redundant pairs of primary and secondary fluid controlchannels, said first redundant pair of primary and secondary fluidcontrol channels including a first primary hydraulic fluid controlchannel and a first secondary hydraulic fluid control channel, each ofsaid first primary and said first secondary hydraulic fluid controlchannels controlling said first set of wheel brakes, said secondredundant pair of primary and secondary fluid control channels includinga second primary hydraulic fluid control channel and a second secondaryhydraulic fluid control channel, each of said second primary and saidsecond secondary hydraulic fluid control channels controlling saidsecond set of wheel brakes, said third redundant pair of primary andsecondary fluid control channels including a third primary hydraulicfluid control channel and a third secondary hydraulic fluid controlchannel, each of said third primary and said third secondary hydraulicfluid control channels controlling said third set of wheel brakes; asecondary hydraulic system connected in fluid communication with saidplurality of wheel brakes for providing hydraulic power for operation ofsaid plurality of wheel brakes in an alternate braking mode, saidsecondary hydraulic system comprising at least one primary hydraulicfluid control channel and at least one secondary hydraulic fluid controlchannel; a control unit for controlling brake pressure communicated tosaid wheel brakes through said primary and secondary hydraulic systems;a monitor channel operatively connected to said primary and secondaryhydraulic systems for detecting faults in said primary and secondaryhydraulic systems and for selecting between the primary and secondaryhydraulic systems for providing braking pressure; and first and secondsolenoid operated shut-off valves operatively connected to said primaryand secondary hydraulic systems, respectively, and to said control unitfor selecting operation of one of said primary and secondary hydraulicsystems, said first and second solenoid operated shut-off valves beingconfigured to operate in a mutually exclusive manner to positivelyselect between operation of said primary and secondary hydraulic systemswithout the possibility of having both systems pressurized at the sametime, wherein said control unit comprises thrust levers, and saidsolenoid operated shut- off valves are implemented through said thrustlevers to positively prevent pressure from the primary hydraulic systemand secondary hydraulic system being applied to the primary or secondaryhydraulic systems during take off; wherein said first and secondsolenoid operated shut-off valves also turn off hydraulic power to saidservo control valves during flight.
 44. The apparatus of claim 36,wherein said first and second solenoid operated shut-off valves comprisethree way solenoid operated shut-off valves.
 45. The apparatus of claim36, wherein said control unit further comprises sensor means fordetermining brake pedal application and for generating a pedalapplication signal indicating when one of said brake pedals has beenapplied.
 46. In combination with an aircraft, an apparatus for dualredundant control of hydraulically operated wheel braking for theaircraft, the aircraft having landing gear that can move between aretracted position and an actuated position, the landing gear having aplurality of wheels and a corresponding plurality of wheel brakes forthe plurality of wheels, wherein said plurality of wheel brakes aresymmetrically divided into three sets of wheel brakes, and a pluralityof brake pedals for controlling operation of braking of said pluralityof said wheels, said apparatus comprising: a primary hydraulic systemconnected in fluid communication with said plurality of wheel brakes forproviding hydraulic power for normal operation of said plurality ofwheel brakes in a normal braking mode, said primary hydraulic systemcomprising at least one primary hydraulic fluid control channel and atleast one secondary hydraulic fluid control channel, the primary andsecondary hydraulic fluid control channels operating simultaneously andautonomously, and wherein said primary hydraulic system comprises first,second and third redundant pairs of primary and secondary fluid controlchannels, said first redundant pair of primary and secondary fluidcontrol channels including a first primary hydraulic fluid controlchannel and a first secondary hydraulic fluid control channel, each ofsaid first primary and said first secondary hydraulic fluid controlchannels controlling said first set of wheel brakes, said secondredundant pair of primary and secondary fluid control channels includinga second primary hydraulic fluid control channel and a second secondaryhydraulic fluid control channel, each of said second primary and saidsecond secondary hydraulic fluid control channels controlling saidsecond set of wheel brakes, said third redundant pair of primary andsecondary fluid control channels including a third primary hydraulicfluid control channel and a third secondary hydraulic fluid controlchannel, each of said third primary and said third secondary hydraulicfluid control channels controlling said third set of wheel brakes; asecondary hydraulic system connected in fluid communication with saidplurality of wheel brakes for providing hydraulic power for operation ofsaid plurality of wheel brakes in an alternate braking mode, saidsecondary hydraulic system comprising at least one primary hydraulicfluid control channel and at least one secondary hydraulic fluid controlchannel; a control unit for controlling brake pressure communicated tosaid wheel brakes through said primary and secondary hydraulic systems;a monitor channel operatively connected to said primary and secondaryhydraulic systems for detecting faults in said primary and secondaryhydraulic systems and for selecting between the primary and secondaryhydraulic systems for providing braking pressure; first and secondsolenoid operated shut-off valves operatively connected to said primaryand secondary hydraulic systems, respectively, and to said control unitfor selecting operation of one of said primary and secondary hydraulicsystems, said first and second solenoid operated shut-off valves beingconfigured to operate in a mutually exclusive manner to positivelyselect between operation of said primary and secondary hydraulic systemswithout the possibility of having both systems pressurized at the sametime, wherein said control unit comprises thrust levers, and saidsolenoid operated shut-off valves are implemented through said thrustlevers to positively prevent pressure from the primary hydraulic systemand secondary hydraulic system being applied to the primary or secondaryhydraulic systems during take off, and wherein said control unit furthercomprises means for sensing weight on said plurality of wheels and forgenerating a brake inhibit signal when weight is not applied on saidplurality of wheels.
 47. In combination with aircraft landing gear, anapparatus for redundant control of hydraulically operated wheel brakingfor the aircraft landing gear moving between a retracted position and anactuated position, the landing gear having a plurality of wheels and acorresponding plurality of wheel brakes for the plurality of wheels,wherein said plurality of wheel brakes are symmetrically divided intothree sets of wheel brakes, and a plurality of brake pedals forcontrolling operation of braking of said plurality of said wheels, saidapparatus comprising: a primary hydraulic system connected in fluidcommunication with said plurality of wheel brakes for providinghydraulic power for normal operation of said plurality of wheel brakesin a normal braking mode, said primary hydraulic system including aplurality of primary hydraulic fluid control channels and acorresponding plurality of secondary hydraulic fluid control channels,said plurality of secondary hydraulic fluid control channels operatingsimultaneously with and independently from said plurality of primaryhydraulic fluid control channels, and wherein said primary hydraulicsystem comprises first, second and third redundant pairs of primary andsecondary fluid control channels, said first redundant pair of primaryand secondary fluid control channels including a first primary hydraulicfluid control channel and a first secondary hydraulic fluid controlchannel, each of said first primary and said first secondary hydraulicfluid control channels controlling said first set of wheel brakes, saidsecond redundant pair of primary and secondary fluid control channelsincluding a second primary hydraulic fluid control channel and a secondsecondary hydraulic fluid control channel, each of said second primaryand said second secondary hydraulic fluid control channels controllingsaid second set of wheel brakes, said third redundant pair of primaryand secondary fluid control channels including a third primary hydraulicfluid control channel and a third secondary hydraulic fluid controlchannel, each of said third primary and said third secondary hydraulicfluid control channels controlling said third set of wheel brakes; asecondary hydraulic system connected in fluid communication with saidplurality of wheel brakes for providing hydraulic power for operation ofsaid plurality of wheel brakes in an alternate braking mode; a controlunit for controlling brake pressure communicated to said wheel brakesthrough said primary and secondary hydraulic systems; and a monitorchannel operatively connected to said primary hydraulic system fordetecting faults in said primary and secondary hydraulic systems and forselecting between the primary and secondary hydraulic systems forproviding braking pressure.
 48. The apparatus of claim 47, wherein saidprimary and secondary fluid control channels are partitioned among saidplurality of sets of said plurality of wheel brakes so that even if boththe primary and secondary channels for one of said sets of saidplurality of wheel brakes fail to apply pressure, braking will be lostto only a portion of the wheel brakes and the loss will be in asymmetrical pattern, and said secondary hydraulic system comprises aplurality of primary hydraulic fluid control channels and a plurality ofsecondary hydraulic fluid control channels.
 49. The apparatus of claim48, wherein said plurality of wheels of said landing gear comprisesright and left main landing gear, each including a front pair of wheelbrakes and an aft pair of wheel brakes, and wherein wheel braking poweris provided by common fluid channels to adjacent ones of said front andaft wheel brakes to provide protection against asymmetrical wheelbraking.
 50. The apparatus of claim 48, further comprising a pluralityof left brake pedals and a plurality of right brake pedals forcontrolling braking, and wherein said plurality of wheels of saidlanding gear further comprises a center landing gear including a frontpair of wheel brakes and an aft pair of wheel brakes, and wherein saidprimary and secondary fluid control channels control all four wheels ofthe center landing gear.
 51. The apparatus of claim 50, wherein wheelbraking power is provided by common fluid channels to the wheel brakesof the center landing gear on an axle pair basis.
 52. The apparatus ofclaim 48 wherein said plurality of primary hydraulic fluid controlchannels comprises three primary hydraulic fluid control channels, andsaid plurality of secondary hydraulic fluid control channels comprisesthree secondary hydraulic fluid control channels.
 53. The apparatus ofclaim 48, wherein each pair of primary and secondary fluid controlchannels controls the same four wheels.
 54. The apparatus of claim 48,wherein said secondary hydraulic system provides pressure for alternatebraking using dual, independent, closed loop analog control circuits.55. The apparatus of claim 48, wherein said plurality of wheels of saidlanding gear comprises right and left main landing gear and a centerlanding gear, and wherein said secondary hydraulic system provides dualredundant analog brake-by-wire control in the alternate braking mode forthe main and center landing gears of the aircraft.
 56. The apparatus ofclaim 47, wherein said secondary hydraulic system comprises a pluralityof accumulators for providing an alternate supply of hydraulic power foran emergency braking mode in the event that both the primary hydraulicsystem and the secondary hydraulic system are depressurized.
 57. Theapparatus of claim 56, wherein said alternate supply of hydraulic poweris provided for an ultimate braking mode providing braking pressure to aplurality of said wheel brakes.
 58. The apparatus of claim 56, whereinsaid alternate supply of hydraulic power is provided for a parking brakemode providing braking pressure to a plurality of said wheel brakes. 59.The apparatus of claim wherein said monitor channel detects occurrenceof loss of pressure in the primary hydraulic system, if any brake hasunwanted pressure applied, and if a fault is detected on primary orsecondary channels of the primary hydraulic system that affects morethan one wheel brake on each landing gear.
 60. The apparatus of claim47, further comprising first and second solenoid operated shut-offvalves operatively connected to said primary and secondary hydraulicsystems, respectively, and to said control unit for selecting operationof one of said primary and secondary hydraulic systems, said first andsecond solenoid operated shut-off valves being configured to operate ina mutually exclusive manner to positively select between operation ofsaid primary and secondary hydraulic systems without the possibility ofhaving both systems pressurized at the same time.
 61. The apparatus ofclaim 60, wherein said control unit comprises thrust levers, and saidsolenoid operated shut-off valves are implemented through said thrustlevers to positively prevent pressure from the primary hydraulic systemand secondary hydraulic system being applied to the normal or alternatebrake metering systems during take off.
 62. In combination with aircraftlanding gear, an apparatus for redundant control of hydraulicallyoperated wheel braking for the aircraft landing gear moving between aretracted position and an actuated position, the landing gear having aplurality of wheels and a corresponding plurality of wheel brakes forthe plurality of wheels, wherein said plurality of wheel brakes aresymmetrically divided into three sets of wheel brakes, and a pluralityof brake pedals for controlling operation of braking of said pluralityof said wheels, said apparatus comprising: a primary hydraulic systemconnected in fluid communication with said plurality of wheel-brakes forproviding hydraulic power for normal operation of said plurality ofwheel brakes in a normal braking mode, said primary hydraulic systemincluding a plurality of primary hydraulic fluid control channels and acorresponding plurality of secondary hydraulic fluid control channels,said plurality of secondary hydraulic fluid control channels operatingsimultaneously and independently from said plurality of primaryhydraulic fluid control channels, and wherein said primary hydraulicsystem comprises first, second and third redundant pairs of primary andsecondary fluid control channels, said first redundant pair of primaryand secondary fluid control channels including a first primary hydraulicfluid control channel and a first secondary hydraulic fluid controlchannel, each of said first primary and said first secondary hydraulicfluid control channels controlling said first set of wheel brakes, saidsecond redundant pair of primary and secondary fluid control channelsincluding a second primary hydraulic fluid control channel and a secondsecondary hydraulic fluid control channel, each of said second primaryand said second secondary hydraulic fluid control channels controllingsaid second set of wheel brakes, said third redundant pair of primaryand secondary fluid control channels including a third primary hydraulicfluid control channel and a third secondary hydraulic fluid controlchannel, each of said third primary and said third secondary hydraulicfluid control channels controlling said third set of wheel brakes; asecondary hydraulic system connected in fluid communication with saidplurality of wheel brakes for providing hydraulic power for operation ofsaid plurality of wheel brakes in an alternate braking mode; a controlunit for controlling brake pressure communicated to said wheel brakesthrough said primary and secondary hydraulic systems; a monitor channeloperatively connected to said primary hydraulic system for detectingfaults in said primary and secondary hydraulic systems and for selectingbetween the primary and secondary hydraulic systems for providingbraking pressure; first and second solenoid operated shut-off valvesoperatively connected to said primary and secondary hydraulic systems,respectively, and to said control unit for selecting operation of one ofsaid primary and secondary hydraulic systems, said first and secondsolenoid operated shut-off valves being configured to operate in amutually exclusive manner to positively select between operation of saidprimary and secondary hydraulic systems without the possibility ofhaving both systems pressurized at the same time; wherein said controlunit comprises thrust levers and corresponding thrust lever switches,and said solenoid operated shut-off valves are implemented through saidthrust levers to positively prevent pressure from the primary hydraulicsystem and secondary hydraulic system being applied to the normal oralternate brake metering systems during take off; and a landing gearlever controlling retraction of the landing gear, and dual redundantswitches on the landing gear lever to bypass the thrust lever switchesto stop the wheels during climb when the thrust levers are advanced toenable wheel braking upon retraction of the landing gear.
 63. Theapparatus of claim 62, wherein each of the primary brake hydraulic fluidcontrol channels receives a software independent signal that initiatesretraction braking for three seconds or until the nose landing gear isup and locked, whichever happens sooner.
 64. The apparatus of claim 62,wherein an anti-skid function of the normal braking mode is inhibitedduring retraction braking.
 65. In combination with aircraft landinggear, an apparatus for redundant control of hydraulically operated wheelbraking for the aircraft landing gear moving between a retractedposition and an actuated position, the landing gear having a pluralityof wheels and a corresponding plurality of wheel brakes for theplurality of wheels, wherein said plurality of wheel brakes aresymmetrically divided into three sets of wheel brakes, and a pluralityof brake pedals for controlling operation of braking of said pluralityof said wheels, said apparatus comprising: a primary hydraulic systemconnected in fluid communication with said plurality of wheel brakes forproviding hydraulic power for normal operation of said plurality ofwheel brakes in a normal braking mode, said primary hydraulic systemincluding a plurality of primary hydraulic fluid control channels and acorresponding plurality of secondary hydraulic fluid control channels,said plurality of secondary hydraulic fluid control channels operatingsimultaneously and independently from said plurality of primaryhydraulic fluid control channels, and wherein said primary hydraulicsystem comprises first, second and third redundant pairs of primary andsecondary fluid control channels, said first redundant pair of primaryand secondary fluid control channels including a first primary hydraulicfluid control channel and a first secondary hydraulic fluid controlchannel, each of said first primary and said first secondary hydraulicfluid control channels controlling said first set of wheel brakes, saidsecond redundant pair of primary and secondary fluid control channelsincluding a second primary hydraulic fluid control channel and a secondsecondary hydraulic fluid control channel, each of said second primaryand said second secondary hydraulic fluid control channels controllingsaid second set of wheel brakes, said third redundant pair of primaryand secondary fluid control channels including a third primary hydraulicfluid control channel and a third secondary hydraulic fluid controlchannel, each of said third primary and said third secondary hydraulicfluid control channels controlling said third set of wheel brakes; asecondary hydraulic system connected in fluid communication with saidplurality of wheel brakes for providing hydraulic power for operation ofsaid plurality of wheel brakes in an alternate braking mode; a controlunit for controlling brake pressure communicated to said wheel brakesthrough said primary and secondary hydraulic systems; a monitor channeloperatively connected to said primary hydraulic system for detectingfaults in said primary and secondary hydraulic systems and for selectingbetween the primary and secondary hydraulic systems for providingbraking pressure; first and second solenoid operated shut-off valvesoperatively connected to said primary and secondary hydraulic systems,respectively, and to said control unit for selecting operation of one ofsaid primary and secondary hydraulic systems, said first and secondsolenoid operated shut-off valves being configured to operate in amutually exclusive manner to positively select between operation of saidprimary and secondary hydraulic systems without the possibility ofhaving both systems pressurized at the same time; wherein said controlunit comprises thrust levers and corresponding thrust lever switches,and said solenoid operated shut-off valves are implemented through saidthrust levers to positively prevent pressure from the primary hydraulicsystem and secondary hydraulic system being applied to the normal oralternate brake metering systems during take off; and wherein saidcontrol unit comprises a plurality of servo control valves controlled bycorresponding dual solenoid coils for controlling the operation of saidwheel brakes, respectively, and wherein said thrust levers comprise dualthrust lever switches that break both power and ground to said first andsecond solenoid operated shut-off valves for the primary hydraulicsystem and secondary hydraulic system when a thrust lever is advanced.66. The apparatus of claim 61, wherein said control unit furthercomprises a plurality of sensors for sensing the position of each brakepedal.
 67. In combination with aircraft landing gear, an apparatus forredundant control of hydraulically operated wheel braking for theaircraft landing gear moving between a retracted position and anactuated position, the landing gear having a plurality of wheels and acorresponding plurality of wheel brakes for the plurality of wheels,wherein said plurality of wheel brakes are symmetrically divided intothree sets of wheel brakes, and a plurality of brake pedals forcontrolling operation of braking of said plurality of said wheels, saidapparatus comprising: a primary hydraulic system connected in fluidcommunication with said plurality of wheel brakes for providinghydraulic power for normal operation of said plurality of wheel brakesin a normal braking mode, said primary hydraulic system including aplurality of primary hydraulic fluid control channels and acorresponding plurality of secondary hydraulic fluid control channels,said plurality of secondary hydraulic fluid control channels operatingsimultaneously and independently from said plurality of primaryhydraulic fluid control channels, and wherein said primary hydraulicsystem comprises first, second and third redundant pairs of primary andsecondary fluid control channels, said first redundant pair of primaryand secondary fluid control channels including a first primary hydraulicfluid control channel and a first secondary hydraulic fluid controlchannel, each of said first primary and said first secondary hydraulicfluid control channels controlling said first set of wheel brakes, saidsecond redundant pair of primary and secondary fluid control channelsincluding a second primary hydraulic fluid control channel and a secondsecondary hydraulic fluid control channel, each of said second primaryand said second secondary hydraulic fluid control channels controllingsaid second set of wheel brakes, said third redundant pair of primaryand secondary fluid control channels including a third primary hydraulicfluid control channel and a third secondary hydraulic fluid controlchannel, each of said third primary and said third secondary hydraulicfluid control channels controlling said third set of wheel brakes; asecondary hydraulic system connected in fluid communication with saidplurality of wheel brakes for providing hydraulic power for operation ofsaid plurality of wheel brakes in an alternate braking mode; a controlunit for controlling brake pressure communicated to said wheel brakesthrough said primary and secondary hydraulic systems; a monitor channeloperatively connected to said primary hydraulic system for detectingfaults in said primary and secondary hydraulic systems and for selectingbetween the primary and secondary hydraulic systems for providingbraking pressure; first and second solenoid operated shut-off valvesoperatively connected to said primary and secondary hydraulic systems,respectively, and to said control unit for selecting operation of one ofsaid primary and secondary hydraulic systems, said first and secondsolenoid operated shut-off valves being configured to operate in amutually exclusive manner to positively select between operation of saidprimary and secondary hydraulic systems without the possibility ofhaving both systems pressurized at the same time; wherein said controlunit comprises thrust levers and corresponding thrust lever switches,and said solenoid operated shut-off valves are implemented through saidthrust levers to positively prevent pressure from the primary hydraulicsystem and secondary hydraulic system being applied to the normal oralternate brake metering systems during take off; wherein said controlunit further comprises a plurality of sensors for sensing the positionof each brake pedal, and wherein said sensors comprise dual redundantswitches that break both power and ground to said first and secondsolenoid operated shut-off valves, such that depression of either brakepedal opens the first and second solenoid operated shut-off valve forthe active hydraulic system.
 68. In combination with aircraft landinggear, an apparatus for redundant control of hydraulically operated wheelbraking for the aircraft landing gear moving between a retractedposition and an actuated position, the landing gear having a pluralityof wheels and a corresponding plurality of wheel brakes for theplurality of wheels, wherein said plurality of wheel brakes aresymmetrically divided into three sets of wheel brakes, and a pluralityof brake pedals for controlling operation of braking of said pluralityof said wheels, said apparatus comprising: a primary hydraulic systemconnected in fluid communication with said plurality of wheel brakes forproviding hydraulic power for normal operation of said plurality ofwheel brakes in a normal braking mode, said primary hydraulic systemincluding a plurality of primary hydraulic fluid control channels and acorresponding plurality of secondary hydraulic fluid control channels,said plurality of secondary hydraulic fluid control channels operatingsimultaneously and independently from said plurality of primaryhydraulic fluid control channels, and wherein said primary hydraulicsystem comprises first, second and third redundant pairs of primary andsecondary fluid control channels, said first redundant pair of primaryand secondary fluid control channels including a first primary hydraulicfluid control channel and a first secondary hydraulic fluid controlchannel, each of said first primary and said first secondary hydraulicfluid control channels controlling said first set of wheel brakes, saidsecond redundant pair of primary and secondary fluid control channelsincluding a second primary hydraulic fluid control channel and a secondsecondary hydraulic fluid control channel, each of said second primaryand said second secondary hydraulic fluid control channels controllingsaid second set of wheel brakes, said third redundant pair of primaryand secondary fluid control channels including a third primary hydraulicfluid control channel and a third secondary hydraulic fluid controlchannel, each of said third primary and said third secondary hydraulicfluid control channels controlling said third set of wheel brakes; asecondary hydraulic system connected in fluid communication with saidplurality of wheel brakes for providing hydraulic power for operation ofsaid plurality of wheel brakes in an alternate braking mode; a controlunit for controlling brake pressure communicated to said wheel brakesthrough said primary and secondary hydraulic systems; a monitor channeloperatively connected to said primary hydraulic system for detectingfaults in said primary and secondary hydraulic systems and for selectingbetween the primary and secondary hydraulic systems for providingbraking pressure; first and second solenoid operated shut-off valvesoperatively connected to said primary and secondary hydraulic systems,respectively, and to said control unit for a selecting operation of oneof said primary and secondary hydraulic systems, said first and secondsolenoid operated shut-off valves being configured to operate in amutually exclusive manner to positively select between operation of saidprimary and secondary hydraulic systems without the possibility ofhaving both systems pressurized at the same time; wherein said controlunit comprises thrust levers and corresponding thrust lever switches,and said solenoid operated shut-off valves are implemented through saidthrust levers to positively prevent pressure from the primary hydraulicsystem and secondary hydraulic system being applied to the normal oralternate brake metering systems during take off; and wherein said firstand second solenoid operated shut-off valves also turn off hydraulicpower to said servo control valves during flight.
 69. The apparatus ofclaim 61, wherein said first and second solenoid operated shut-offvalves comprise three way solenoid operated shut-off valves.
 70. Theapparatus of claim 61, wherein said control unit further comprisessensor means for determining brake pedal application and for generatinga pedal application signal indicating actuation of the wheel brakingsystem when said brake pedal has been applied.
 71. In combination withaircraft landing gear, an apparatus for redundant control ofhydraulically operated wheel braking for the aircraft landing gearmoving between a retracted position and an actuated position, thelanding gear having a plurality of wheels and a corresponding pluralityof wheel brakes for the plurality of wheels, wherein said plurality ofwheel brakes are symmetrically divided into three sets of wheel brakes,and a plurality of brake pedals for controlling operation of braking ofsaid plurality of said wheels, said apparatus comprising: a primaryhydraulic system connected in fluid communication with said plurality ofwheel brakes for providing hydraulic power for normal operation of saidplurality of wheel brakes in a normal braking mode, said primaryhydraulic system including a plurality of primary hydraulic fluidcontrol channels and a corresponding plurality of secondary hydraulicfluid control channels, said plurality of secondary hydraulic fluidcontrol channels operating simultaneously and independently from saidplurality of primary hydraulic fluid control channels, and wherein saidprimary hydraulic system comprises first, second and third redundantpairs of primary and secondary fluid control channels, said firstredundant pair of primary and secondary fluid control channels includinga first primary hydraulic fluid control channel and a first secondaryhydraulic fluid control channel, each of said first primary and saidfirst secondary hydraulic fluid control channels controlling said firstset of wheel brakes, said second redundant pair of primary and secondaryfluid control channels including a second primary hydraulic fluidcontrol channel and a second secondary hydraulic fluid control channel,each of said second primary and said second secondary hydraulic fluidcontrol channels controlling said second set of wheel brakes, said thirdredundant pair of primary and secondary fluid control channels includinga third primary hydraulic fluid control channel and a third secondaryhydraulic fluid control channel, each of said third primary and saidthird secondary hydraulic fluid control channels controlling said thirdset of wheel brakes; a secondary hydraulic system connected in fluidcommunication with said plurality of wheel brakes for providinghydraulic power for operation of said plurality of wheel brakes in analternate braking mode; a control unit for controlling brake pressurecommunicated to said wheel brakes through said primary and secondaryhydraulic systems; a monitor channel operatively connected to saidprimary hydraulic system for detecting faults in said primary andsecondary hydraulic systems and for selecting between the primary andsecondary hydraulic systems for providing braking pressure; first andsecond solenoid operated shut-off valves operatively connected to saidprimary and secondary hydraulic systems, respectively, and to saidcontrol unit for selecting operation of one of said primary andsecondary hydraulic systems, said first and second solenoid operatedshut-off valves being configured to operate in a mutually exclusivemanner to positively select between operation of said primary andsecondary hydraulic systems without the possibility of having bothsystems pressurized at the same time; wherein said control unitcomprises thrust levers and corresponding thrust lever switches, andsaid solenoid operated shut-off valves are implemented through saidthrust levers to positively prevent pressure from the primary hydraulicsystem and secondary hydraulic system being applied to the normal oralternate brake metering systems during take off; and wherein saidcontrol unit further comprises means for sensing weight on said wheeland for generating a brake inhibit signal when weight is not applied onsaid wheel.
 72. The apparatus of claim 47, wherein said control unitfurther comprises a plurality of servo control valves for controllingthe operation of said wheel brakes, and wherein said plurality of servocontrol valves are controlled by corresponding dual solenoid coils. 73.The apparatus of claim 72, wherein said plurality of servo controlvalves are further controlled with pressure feedback.
 74. The apparatusof claim 72, wherein said plurality of servo control valves comprisestwelve dual coil brake control valves in the primary hydraulic fluidsystem.
 75. The apparatus of claim 73, further comprising antiskidcontrol provided on each wheel brake utilizing the dual coil servocontrol valves with pressure feedback.
 76. In combination with aircraftlanding gear, an apparatus for redundant control of hydraulicallyoperated wheel braking for the aircraft landing gear moving between aretracted position and an actuated position, the landing gear having aplurality of wheels and a corresponding plurality of wheel brakes forthe plurality of wheels, wherein said plurality of wheel brakes aresymmetrically divided into three sets of wheel brakes, and a pluralityof brake pedals for controlling operation of braking of said pluralityof said wheels, said apparatus comprising: a primary hydraulic systemconnected in fluid communication with said plurality of wheel brakes forproviding hydraulic power for normal operation of said plurality ofwheel brakes in a normal braking mode, said primary hydraulic systemincluding a plurality of primary hydraulic fluid control channels and acorresponding plurality of secondary hydraulic fluid control channels,said plurality of secondary hydraulic fluid control channels operatingsimultaneously and independently from said plurality of primaryhydraulic fluid control channels, and wherein said primary hydraulicsystem comprises first, second and third redundant pairs of primary andsecondary fluid control channels, said first redundant pair of primaryand secondary fluid control channels including a first primary hydraulicfluid control channel and a first secondary hydraulic fluid controlchannel, each of said first primary and said first secondary hydraulicfluid control channels controlling said first set of wheel brakes, saidsecond redundant pair of primary and secondary fluid control channelsincluding a second primary hydraulic fluid control channel and a secondsecondary hydraulic fluid control channel, each of said second primaryand said second secondary hydraulic fluid control channels controllingsaid second set of wheel brakes, said third redundant pair of primaryand secondary fluid control channels including a third primary hydraulicfluid control channel and a third secondary hydraulic fluid controlchannel, each of said third primary and said third secondary hydraulicfluid control channels controlling said third set of wheel brakes; asecondary hydraulic system connected in fluid communication with saidplurality of wheel brakes for providing hydraulic power for operation ofsaid plurality of wheel brakes in an alternate braking mode; a controlunit for controlling brake pressure communicated to said wheel brakesthrough said primary and secondary hydraulic systems; a monitor channeloperatively connected to said primary hydraulic system for detectingfaults in said primary and secondary hydraulic systems and for selectingbetween the primary and secondary hydraulic systems for providingbraking pressure; and wherein said control unit comprises system sixbrake control cards providing dual redundant digital brake-by-wirecontrols.
 77. The apparatus of claim 47, wherein said control unitcomprises a built-in-test equipment card for said primary hydraulicfluid system providing mutually exclusive hydraulic system selection.78. The apparatus of claim 72, wherein said control unit comprises apressure sensor mounted downstream of each servo control valve fordetecting asymmetrical braking due to unwanted pressure applied to anywheel brake, whereby the monitor channel can select the alternatehydraulic fluid system.
 79. The apparatus of claim 47, furthercomprising an autobrake control subsystem.
 80. The apparatus of claim50, wherein said primary and secondary hydraulic fluid control channelsare connected to said front and aft pairs of wheel brakes such that theleft brake pedals control the front axle pair and the right brake pedalscontrol the aft axle pair.
 81. The apparatus of claim 56, wherein saidemergency braking system provides braking to center landing gear andmain landing gear wheel brakes.